Communication device and non-transitory computer readable storage medium

ABSTRACT

A communication device comprising: a plurality of wireless communication sections; and a control section configured to repeatedly perform a measurement process including transmission of a signal from a representative wireless communication section, reception of the signal from another communication device by the plurality of wireless communication sections, and calculation of a reliability parameter with regard to at least any of the wireless communication sections, control a selection process of selecting the representative wireless communication section each time the measurement process is repeated, and control a positional parameter determination process on a basis of the reliability parameter, the positional parameter determination process being a process of determining a positional parameter indicating a position of the other communication device on a basis of a plurality of a first incoming waves obtained through repetition of the measurement process.

CROSS REFERENCE TO RELATED APPLICATION(S)

This application is based upon and claims benefit of priority fromJapanese Patent Application No. 2020-139211, filed on Aug. 20, 2020, theentire contents of which are incorporated herein by reference.

BACKGROUND

The present invention relates to a communication device and anon-transitory computer readable storage medium.

In recent years, technologies that allow one device to estimate aposition of another device in accordance with a result oftransmitting/receiving a signal between the devices have been developed.As an example of the technologies of determining a position, WO2015/176776 A1 discloses a technology that allows an UWB(ultra-wideband) receiver to estimate an angle of incidence of awireless signal from an UWB transmitter by performing wirelesscommunication using UWB.

However, the technology disclosed by WO 2015/176776 A1 does not dealwith reduction in accuracy of estimating the angle of incidence of thewireless signal in an environment where an obstacle is interposedbetween the transmitter and the receiver, or other environments. Inaddition to dealing with the above-described issue, it has been desiredto improve accuracy of the position estimation technologies more.

Accordingly, the present invention is made in view of the aforementionedissues, and an object of the present invention is to provide a mechanismthat makes it possible to control a process of estimating a position inaccordance with a radio propagation environment.

SUMMARY

To solve the above described problem, according to an aspect of thepresent invention, there is provided a communication device comprising:a plurality of wireless communication sections, each of which isconfigured to be capable of wirelessly transmitting and receiving asignal to and from another communication device; and a control sectionconfigured to repeatedly perform a measurement process includingtransmission of a signal from a representative wireless communicationsection that is a wireless communication section selected from among theplurality of wireless communication sections, reception of the signal bythe plurality of wireless communication sections, and calculation of afirst parameter belonging to a reliability parameter with regard to atleast any of the wireless communication sections, the reliabilityparameter serving as an indicator that indicates whether a firstincoming wave is appropriate for a processing target, the first incomingwave being a signal detected as a signal that meets a predetermineddetection standard among the signals received by the wirelesscommunication section, control a selection process of selecting therepresentative wireless communication section from among the pluralityof wireless communication sections each time the measurement process isrepeated, and control a positional parameter determination process on abasis of the first parameter, the positional parameter determinationprocess being a process of determining a positional parameter indicatinga position of the other communication device on a basis of a pluralityof the first incoming waves obtained through repetition of themeasurement process.

To solve the above described problem, according to another aspect of thepresent invention, there is provided a non-transitory computer readablestorage medium having a program that causes a computer for controlling acommunication device including a plurality of wireless communicationsections, each of which is configured to be capable of wirelesslytransmitting and receiving a signal to and from another communicationdevice, to function as a control section configured to repeatedlyperform a measurement process including transmission of a signal from arepresentative wireless communication section that is a wirelesscommunication section selected from among the plurality of wirelesscommunication sections, reception of the signal by the plurality ofwireless communication sections, and calculation of a first parameterbelonging to a reliability parameter with regard to at least any of thewireless communication sections, the reliability parameter serving as anindicator that indicates whether a first incoming wave is appropriatefor a processing target, the first incoming wave being a signal detectedas a signal that meets a predetermined detection standard among thesignals received by the wireless communication section, control aselection process of selecting the representative wireless communicationsection from among the plurality of wireless communication sections eachtime the measurement process is repeated, and control a positionalparameter determination process on a basis of the first parameter, thepositional parameter determination process being a process ofdetermining a positional parameter indicating a position of the othercommunication device on a basis of a plurality of the first incomingwaves obtained through repetition of the measurement process.

Accordingly, the present invention is made in view of the aforementionedissues, and an object of the present invention is to provide a mechanismthat makes it possible to control a process of estimating a position inaccordance with a radio propagation environment.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating an example of a configuration of asystem according to an embodiment of the present invention.

FIG. 2 is a diagram illustrating an example of arrangement of aplurality of antennas installed in a vehicle according to theembodiment.

FIG. 3 is a diagram illustrating an example of a positional parameter ofa portable device according to the embodiment.

FIG. 4 is a diagram illustrating an example of the positional parameterof the portable device according to the embodiment.

FIG. 5 is a diagram illustrating an example of processing blocks forsignal processing in a communication unit according to the embodiment.

FIG. 6 is a graph illustrating an example of a CIR according to theembodiment.

FIG. 7 is a sequence diagram illustrating an example of a flow of aranging process performed in the system according to the embodiment.

FIG. 8 is a sequence diagram illustrating an example of a flow of anangle estimation process performed in the system according to theembodiment.

FIG. 9 is a graph illustrating an example of a CIR with regard to awireless communication section in an LOS condition.

FIG. 10 is a graph illustrating an example of a CIR with regard to thewireless communication section in an NLOS condition.

FIG. 11 is a graph illustrating an example of a CIR with regard to thewireless communication section in the LOS condition.

FIG. 12 is a graph illustrating an example of a CIR with regard to thewireless communication section in the NLOS condition.

FIG. 13 is graphs illustrating an example of CIRs with regard to aplurality of the wireless communication sections.

FIG. 14 is graphs illustrating an example of CIRs with regard to theplurality of wireless communication sections.

FIG. 15 is diagrams for describing examples of a reliability parameteraccording to the embodiment.

FIG. 16 is diagrams for describing examples of the reliability parameteraccording to the embodiment.

FIG. 17 is a graph illustrating an example of a CIR.

FIG. 18 is a graph illustrating an example of a CIR.

FIG. 19 is a flowchart illustrating an example of a flow of a processperformed by the communication unit of the vehicle according to theembodiment.

DETAILED DESCRIPTION OF THE EMBODIMENT(S)

Hereinafter, referring to the appended drawings, preferred embodimentsof the present invention will be described in detail. It should be notedthat, in this specification and the appended drawings, structuralelements that have substantially the same function and structure aredenoted with the same reference numerals, and repeated explanationthereof is omitted.

Further, in the present specification and the drawings, differentalphabets are suffixed to a same reference numeral to distinguishelements which have substantially the same functional configuration. Forexample, a plurality of elements which have substantially the samefunctional configuration are distinguished such as wirelesscommunication sections 210A, 210B, and 210C, as necessary. However, whenthere is no need in particular to distinguish elements that havesubstantially the same functional configuration, the same referencenumeral alone is attached. For example, in the case where it is notnecessary to particularly distinguish the wireless communicationsections 210A, 210B, and 210C, the wireless communication sections 210A,210B, and 210C are simply referred to as the wireless communicationsections 210.

1. Configuration Example

FIG. 1 is a diagram illustrating an example of a configuration of asystem 1 according to an embodiment of the present invention. Asillustrated in FIG. 1, the system 1 according to the present embodimentincludes a portable device 100 and a communication unit 200. Thecommunication unit 200 according to the present embodiment is installedin a vehicle 202. The vehicle 202 is an example of a usage target of theuser.

A communication device of an authenticatee and a communication device ofan authenticator are involved in the present embodiment. In the exampleillustrated in FIG. 1, the portable device 100 is an example of thecommunication device of the authenticatee, and the communication unit200 is an example of the communication device of the authenticator.

When a user (for example, a driver of the vehicle 202) carrying theportable device 100 approaches the vehicle 202, the system 1 performswireless communication for authentication between the portable device100 and the communication unit 200 installed in the vehicle 202. Next,when the authentication succeeds, the vehicle 202 becomes available forthe user by unlocking a door lock of the vehicle 202 and starting anengine of the vehicle 202. The system 1 is also referred to as a smartentry system. Next, respective structural elements will be describedsequentially.

(1) Portable Device 100

The portable device 100 is configured as any device to be carried by theuser. Examples of the any device include an electronic key, asmartphone, a wearable terminal, and the like. As illustrated in FIG. 1,the portable device 100 includes a wireless communication section 110, astorage section 120, and a control section 130.

The wireless communication section 110 has a function of performingwireless communication with the communication unit 200 installed in thevehicle 202. The wireless communication section 110 wirelessly receivesa signal from the communication unit 200 installed in the vehicle 202.In addition, the wireless communication section 110 wirelessly transmitsa signal to the communication unit 200.

For example, wireless communication is performed between the wirelesscommunication section 110 and the communication unit 200 by using anultra-wideband (UWB) signal, for example. In the wireless communicationof the UWB signal, it is possible for impulse UWB to measure propagationdelay time of a radio wave with high accuracy by using the radio wave ofultra-short pulse width of a nanosecond or less, and it is possible toperform ranging with high accuracy on the basis of the propagation delaytime. Note that, the propagation delay time is time from transmission toreception of the radio wave. The wireless communication section 110 isconfigured as a communication interface that makes it possible toperform communication by using the UWB signals, for example.

Note that, the UWB signal may be transmitted/received as a rangingsignal, an angle estimation signal, and a data signal, for example. Theranging signal is a signal transmitted and received in the rangingprocess (to be described later). The ranging signal may be configured ina frame format that does not include a payload part for storing data orin a frame format that includes the payload part. The angle estimationsignal is a signal transmitted and received in an angle estimationprocess (to be described later). The angle estimation signal may beconfigured in a way similar to the ranging signal. The data signal ispreferably configured in the frame format that includes the payload partfor storing the data.

Here, the wireless communication section 110 includes at least oneantenna 111. In addition, the wireless communication section 110transmits/receives a wireless signal via the at least one antenna 111.

The storage section 120 has a function of storing various kinds ofinformation for operating the portable device 100. For example, thestorage section 120 stores a program for operating the portable device100, and an identifier (ID), password, and authentication algorithm forauthentication, and the like. For example, the storage section 120includes a storage medium such as flash memory and a processing devicethat performs recording/playback on/of the storage medium.

The control section 130 has a function of executing processes in theportable device 100. For example, the control section 130 controls thewireless communication section 110 to perform communication with thecommunication unit 200 of the vehicle 202. The control section 130 readsinformation from the storage section 120 and writes information into thestorage section 120. The control section 130 also functions as anauthentication control section that controls an authentication processbetween the portable device 100 and the communication unit 200 of thevehicle 202. For example, the control section 130 may include a centralprocessing unit (CPU) and an electronic circuit such as amicroprocessor.

(2) Communication Unit 200

The communication unit 200 is prepared in association with the vehicle202. Here, it is assumed that the communication unit 200 is installed inthe vehicle 202 in such a manner that the communication section 200 isinstalled in a vehicle interior of the vehicle 202, the communicationsection 200 is built in the vehicle 202 as a communication module, or inother manners. Alternatively, the communication unit 200 may be preparedas a separate object from the vehicle 202 in such a manner that thecommunication unit 200 is installed in a parking space for the vehicle202 or in other manners. In this case, the communication unit 200 maywirelessly transmit a control signal to the vehicle 202 on the basis ofa result of communication with the portable device 100 and may remotelycontrol the vehicle 202. As illustrated in FIG. 1, the communicationunit 200 includes a plurality of wireless communication sections 210(210A to 210D), a storage section 220, and a control section 230.

The wireless communication section 210 has a function of performingwireless communication with the wireless communication section 110 ofthe portable device 100. The wireless communication section 210wirelessly receives a signal from the portable device 100. In addition,the wireless communication section 210 wirelessly transmits a signal tothe portable device 100. The wireless communication section 210 isconfigured as a communication interface that makes it possible toperform communication by using the UWB, for example.

Here, each of the wireless communication sections 210 includes anantenna 211. In addition, each of the wireless communication sections210 transmits/receives a wireless signal via the antenna 211.

The storage section 220 has a function of storing various kinds ofinformation for operating the communication unit 200. For example, thestorage section 220 stores a program for operating the communicationunit 200, an authentication algorithm, and the like. For example, thestorage section 220 includes a storage medium such as flash memory and aprocessing device that performs recording/playback on/of the storagemedium.

The control section 230 has a function of controlling overall operationperformed by the communication unit 200 and in-vehicle equipmentinstalled in the vehicle 202. For example, the control section 230controls the wireless communication sections 210 to performcommunication with the portable device 100. The control section 230reads information from the storage section 220 and writes informationinto the storage section 220. The control section 230 also functions asan authentication control section that controls the authenticationprocess between the portable device 100 and the communication unit 200of the vehicle 202. In addition, the control section 230 also functionsas a door lock control section that controls a door lock of the vehicle202, and opens and closes the door lock. The control section 230 alsofunctions as an engine control section that controls the engine of thevehicle 202, and starts/stops the engine. Note that, a motor or the likemay be installed as a power source in the vehicle 202 in addition to theengine. For example, the control section 230 is configured as anelectronic circuit such as an electronic control unit (ECU).

2. Estimation of Positional Parameter

<2.1. Positional Parameter>

The communication unit 200 (specifically, control section 230) accordingto the present embodiment performs a positional parameter estimationprocess of estimating a positional parameter that represents a positionof the portable device 100. Hereinafter, with reference to FIG. 2 toFIG. 4, various definitions related to the positional parameter will bedescribed.

FIG. 2 is a diagram illustrating an example of arrangement of theplurality of antennas 211 (wireless communication sections 210)installed in the vehicle 202 according to the present embodiment. Asillustrated in FIG. 2, the four antennas 211 (211A to 211D) areinstalled on a ceiling of the vehicle 202. The antenna 211A is installedon a front right side of the vehicle 202. The antenna 211B is installedon a front left side of the vehicle 202. The antenna 211C is installedon a rear right side of the vehicle 202. The antenna 211D is installedon a rear left side of the vehicle 202. Note that, distances betweenadjacent antennas 211 are set to a half or less of wavelength λ of acarrier wave of an angle estimation signal (to be described later). Alocal coordinate system of the communication unit 200 is set as acoordinate system based on the communication unit 200. An example of thelocal coordinate system of the communication unit 200 has its origin atthe center of the four antennas 211. This local coordinate system hasits X axis along a front-rear direction of the vehicle 202, its Y axisalong a left-right direction of the vehicle 202, and its Z axis along anup-down direction of the vehicle 202. Note that, the X axis is parallelto a line connecting a pair of the antennas in the front-rear direction(such as a pair of the antenna 211A and the antenna 211C, and a pair ofthe antenna 211B and the antenna 211D). In addition, the Y axis isparallel to a line connecting a pair of the antennas in the left-rightdirection (such as a pair of the antenna 211A and the antenna 211B, anda pair of the antenna 211C and the antenna 211D).

Note that, the arrangement of the four antennas 211 is not limited tothe square shape. The arrangement of the four antennas 211 may be aparallelogram shape, a trapezoid shape, a rectangular shape, or anyother shapes. Of course, the number of antennas 211 is not limited tofour.

FIG. 3 is a diagram illustrating an example of a positional parameter ofthe portable device 100 according to the present embodiment. Thepositional parameter may include a distance R between the portabledevice 100 and the communication unit 200. The distance R illustrated inFIG. 3 is a distance from the origin of the local coordinate system ofthe communication unit 200 to the portable device 100. The distance R isestimated on the basis of a result of transmission/reception of aranging signal (to be described later) between the portable device 100and one of the plurality of wireless communication sections 210. Thedistance R may be a distance between the portable device 100 and one ofthe wireless communication sections 210 that transmit/receive theranging signal (to be described later).

In addition, as illustrated in FIG. 3, the positional parameter mayinclude an angle of the portable device 100 based on the communicationunit 200, the angle including an angle α between the X axis and theportable device 100 and an angle β between the Y axis and the portabledevice 100. The angles α and β are angles between the coordinate axes ofa first predetermined coordinate system and a straight line connectingthe portable device 100 with the origin of the first predeterminedcoordinate system. For example, the first predetermined coordinatesystem is the local coordinate system of the communication unit 200. Theangle α is an angle between the X axis and the straight line connectingthe portable device 100 with the origin. The angle β is an angle betweenthe Y axis and the straight line connecting the portable device 100 withthe origin.

FIG. 4 is a diagram illustrating an example of the positional parameterof the portable device 100 according to the present embodiment. Thepositional parameter may include coordinates of the portable device 100in a second predetermined coordinate system. In FIG. 4, a coordinate xon the X axis, a coordinate y on the Y axis, and a coordinate z on the Zaxis of the portable device 100 are an example of such coordinates. Inother words, the second predetermined coordinate system may be the localcoordinate system of the communication unit 200. Alternatively, thesecond predetermined coordinate system may be a global coordinatesystem.

<2.2. CIR>

(1) CIR Calculation Process

In the positional parameter estimation process, the portable device 100and the communication unit 200 communicate with each other to estimatethe positional parameter. At this time, the portable device 100 and thecommunication unit 200 calculates channel impulse responses (CIRs).

The CIR is a response obtained when an impulse is input to the system.In the case where a wireless communication section of one of theportable device 100 and the communication unit 200 (hereinafter, alsoreferred to as a transmitter) transmits a signal including a pulse as afirst signal, the CIR according to the present embodiment is calculatedon the basis of a second signal that corresponds to the first signal andthat is received by a wireless communication section of the other(hereinafter, also referred to as a receiver). The pulse is a signalincluding change in amplitude. It can be said that the CIR indicatescharacteristics of a wireless communication path between the portabledevice 100 and the communication unit 200. Hereinafter, the first signalis also referred to as a transmission signal, and the second signal isalso referred to as a reception signal.

For example, the CIR may be a correlation computation result that is aresult obtained by correlating the transmission signal with thereception signal at each designated interval. Here, the correlation maybe sliding correlation that is a process of correlating the transmissionsignal with the reception signal by shifting relative positions of thesignals in a time direction. The correlation computation result includesa correlation value indicating a degree of the correlation between thetransmission signal and the reception signal as an element obtained ateach designated interval. The designated interval is an interval betweentimings at which the receiver samples the reception signal. Therefore,an element included in the CIR is also referred to as a sampling point.The correlation value may be a complex number including IQ components.In addition, the correlation value may be a phase or amplitude of thecomplex number. In addition, the correlation value may be electric powerthat is a sum of squares of an I component and a Q component of thecomplex number (or square of amplitude).

The CIR is also considered as a set of elements that are values obtainedat respective times (hereinafter, also referred to as CIR values). Inthis case, the CIR is chronological variation in the CIR value. In thecase where the CIR is the correlation computation result, the CIR valueis the correlation value.

Note that, the portable device 100 and the communication unit 200acquire time by using a time counter. The time counter is a counter thatcounts (typically, increments) a value (hereinafter, also referred to ascount value) indicating elapsed time obtained at a predetermined timeinterval (hereinafter, also referred to as count cycle). A current timeis calculated on the basis of the count value counted by the timecounter, the count cycle, and a count start time. If different deviceshave a same count cycle and a same count start time, this means thatthese devices are in synchronization with each other. On the other hand,if at least any of the count cycle and the count start time is differentbetween the different devices, this means that these devices are not insynchronization with each other or are asynchronous with each other. Theportable device 100 and the communication unit 200 may be insynchronization with each other or asynchronous with each other. Inaddition, the plurality of wireless communication sections 210 may be insynchronization with each other or asynchronous with each other. Thedesignated interval used when calculating the CIR may be an integermultiple of the count cycle of the time counter.

Hereinafter, with reference to FIG. 5, a CIR calculation processperformed in the case where the portable device 100 serves as thetransmitter and the communication unit 200 serves as the receiver willbe described in detail.

FIG. 5 is a diagram illustrating an example of processing blocks forsignal processing in the communication unit 200 according to the presentembodiment. As illustrated in FIG. 5, the communication unit 200includes an oscillator 212, a multiplier 213, a 90-degree phase shifter214, a multiplier 215, a low pass filter (LPF) 216, an LPF 217, acorrelator 218, and an integrator 219.

The oscillator 212 generates a signal of same frequency as frequency ofa carrier wave that carries a transmission signal, and outputs thegenerated signal to the multiplier 213 and the 90-degree phase shifter214.

The multiplier 213 multiplies a reception signal received by the antenna211 by the signal output from the oscillator 212, and outputs a resultof the multiplication to the LPF 216. Among input signals, the LPF 216outputs a signal of lower frequency than the frequency of the carrierwave that carries the transmission signal, to the correlator 218. Thesignal input to the correlator 218 is an I component (that is, a realpart) among components corresponding to an envelope of the receptionsignal.

The 90-degree phase shifter 214 delays the phase of the input signal by90 degrees, and outputs the delayed signal to the multiplier 215. Themultiplier 215 multiplies the reception signal received by the antenna211 by the signal output from the 90-degree phase shifter 214, andoutputs a result of the multiplication to the LPF 217. Among inputsignals, the LPF 217 outputs a signal of lower frequency than thefrequency of the carrier wave that carries the transmission signal, tothe correlator 218. The signal input to the correlator 218 is a Qcomponent (that is, an imaginary part) among the componentscorresponding to the envelope of the reception signal.

The correlator 218 calculates the CIR by correlating a reference signalwith the reception signals including the I component and the Q componentoutput from the LPF 216 and the LPF 217 through the sliding correlation.Note that, the reference signal described herein is the same signal asthe transmission signal before multiplying the carrier wave.

The integrator 219 integrates the CIRs output from the correlator 218,and outputs the integrated CIRs.

Here, the transmitter may transmit a signal including a preamble as thetransmission signal. The preamble is a sequence known to the transmitterand the receiver. Typically, the preamble is arranged at a head of thetransmission signal. The preamble includes one or more preamble symbols.The preamble symbol is a pulse sequence including one or more pulses.The pulse sequence is a set of the plurality of pulses that are separatefrom each other in the time direction.

The preamble symbol is a target of integration performed by theintegrator 219. Therefore, the correlator 218 calculates the CIR foreach of the one or more preamble symbols by correlating a portioncorresponding to a preamble symbol with a preamble symbol included inthe transmission signal with regard to each of portions corresponding tothe one or more preamble symbols included in the reception signal, atthe designated intervals. Next, the integrator 219 obtains integratedCIRs by integrating the CIRs of the respective preamble symbols withregard to the one or more preamble symbols included in the preamble.Next, the integrator 219 outputs the integrated CIRs. Hereinafter, theCIR means the integrated CIRs unless otherwise noted.

(2) Example of CIR

FIG. 6 illustrates an example of the CIR output from the integrator 219.FIG. 6 is a graph illustrating an example of the CIR according to thepresent embodiment. The CIR illustrated in FIG. 6 is a CIR obtained onan assumption that the count start time is time when the transmitter hastransmitted the transmission signal. Such a CIR is also referred to asdelay profile. The graph includes a horizontal axis representing delaytime. The delay time is time elapsed after the time when the transmitterhas transmitted the transmission signal. The graph includes a verticalaxis representing absolute values of CIR values (such as amplitude orelectric power).

Note that, the shape of CIR, more specifically, the shape ofchronological change in the CIR value may also be referred to as a CIRwaveform. Typically, a set of elements obtained between a zero-crossingand another zero-crossing corresponds to a single pulse with regard tothe CIR. The zero-crossings are elements whose value is zero. However,the same does not apply to an environment with noise. For example, a setof elements obtained between intersections of a standard withchronological change in the CIR value may be treated as corresponding tothe single pulse. The CIR illustrated in FIG. 6 includes a set 21 ofelements corresponding to a certain pulse, and a set 22 of elementscorresponding to another pulse.

For example, the set 21 corresponds to a signal (such as pulse) thatreaches the receiver through a first path. The first path is a shortestpath between the transmitter and the receiver. In an environment thatincludes no obstacle. The first path is a straight path between thetransmitter and the receiver. For example, the set 22 corresponds to asignal (such as pulse) that reaches the receiver through a path otherthan the first path. As described above, the signals that have passedthrough different paths are also referred to as multipath waves.

(3) Detection of First Incoming Wave

Among wireless signals received from the transmitter, the receiverdetects a signal that meets a predetermined detection standard as asignal that reaches the receiver through the first path. Next, thereceiver estimates the positional parameters on the basis of thedetected signal. Hereinafter, the signal detected as the signal thatreaches the receiver through the first path is also referred to as thefirst incoming wave.

The receiver detects a signal that meets a predetermined detectionstandard as the first incoming wave, among the received wirelesssignals. For example, the predetermined detection standard is acondition that the CIR value (such as amplitude or electric power)exceeds a predetermined threshold for the first time. In other words,the receiver may detect a signal corresponding to a portion of the CIRobtained when the CIR value exceeds the predetermined threshold for thefirst time, as the first incoming wave. Hereinafter, the predeterminedthreshold used for detecting the first incoming wave is also referred toas a first path threshold.

The signal received by the receiver may be any of a direct wave, adelayed wave, or a combined wave. The direct wave is a signal thatpasses through a shortest path between the transmitter and the receiver,and is received by the receiver. In other words, the direct wave is asignal that reaches the receiver through the first path. The delayedwave is a signal that reaches the receiver through a path different fromthe shortest path between the transmitter and the receiver, that is,through a path other than the first path. The delayed wave is receivedby the receiver after getting delayed in comparison with the directwave. The combined wave is a signal received by the receiver in a stateof combining a plurality of signals that have passed through a pluralityof different paths.

Here, it should be noted that the signal detected as the first incomingwave is not necessarily the direct wave. For example, if the direct waveis received in a state where the direct wave and the delayed waveannihilate each other, sometimes the CIR value of the CIR correspondingto the direct wave falls below the predetermined threshold and thedirect wave is not detected as the first incoming wave. In this case,the combined wave or the delayed wave coming while being delayed behindthe direct wave is detected as the first incoming wave.

Reception Time of First Incoming Wave

The receiver may treat the time of meeting the predetermined detectionstandard as reception time of the first incoming wave. For example, thereception time of the first incoming wave is time corresponding to anelement whose CIR value exceeds the first path threshold for the firsttime, that is, time when the CIR value exceeds the first path thresholdfor the first time.

Alternatively, the receiver may treat time of obtaining a peak of thedetected first incoming wave as the reception time of the first incomingwave. In this case, for example, the reception time of the firstincoming wave is time corresponding to an element having highestamplitude or electric power as the CIR value, among the set of elementscorresponding to the first incoming wave with regard to the CIR.

Hereinafter, it is assumed that the reception time of the first incomingwave is time corresponding to an element having a CIR value that exceedsthe first path threshold for the first time.

Phase of First Incoming Wave

The receiver may treat a phase obtained at time of meeting thepredetermined detection standard as a phase of the first incoming wave.For example, the phase of the first incoming wave is a phase serving asa CIR value of an element having the CIR value that exceeds the firstpath threshold for the first time.

Alternatively, the receiver may treat a phase of the peak of thedetected first incoming wave as the phase of the first incoming wave. Inthis case, for example, the reception time of the first incoming wave isthe phase serving as a CIR value of an element having highest amplitudeor electric power as the CIR value, among the set of elementscorresponding to the first incoming wave with regard to the CIR.

Hereinafter, it is assumed that the phase of the first incoming wave isa phase serving as a CIR value of an element having the CIR value thatexceeds the first path threshold for the first time.

Width of First Incoming Wave

The width of the set of elements corresponding to the first incomingwave in the time direction is also referred to as the width of the firstincoming wave. For example, the width of the first incoming wave is thewidth between a zero-crossing and another zero-crossing of the CIR inthe time direction. For another example, the width of the first incomingwave is width between intersections of a standard with chronologicalchange in the CIR value in the time direction.

In the case where only the direct wave is detected as the first incomingwave, the first incoming wave of the CIR has an ideal width. The idealwidth obtained when only the direct wave is detected as the firstincoming wave can be calculated through theoretical calculation usingthe waveform of the transmission signal, a reception signal processingmethod, and the like. On the other hand, in the case where the combinedwave is received as the first incoming wave, the width of the firstincoming wave of the CIR may be different from the ideal width. Forexample, in the case where a combined wave obtained by combining adirect wave and a delayed wave having a same phase as the direct wave isdetected as the first incoming wave, a portion corresponding to thedirect wave and a portion corresponding to the delayed wave are added ina state where they are shifted in the time direction. Therefore, theportions reinforce each other, and the first incoming wave in the CIRhas a wider width. On the other hand, in the case where a combined waveobtained by combining a direct wave and a delayed wave having anopposite phase from the direct wave is detected as the first incomingwave, the direct wave and the delayed wave annihilate each other.Therefore, the first incoming wave in the CIR has a narrower width.

<2.3. Estimation of Positional Parameter>

(1) Ranging

The communication unit 200 performs the ranging process. The rangingprocess is a process of estimating a distance between the communicationunit 200 and the portable device 100. For example, the distance betweenthe communication unit 200 and the portable device 100 is the distance Rillustrated in FIG. 3. The ranging process includestransmission/reception of a ranging signal and calculation of thedistance R based on propagation delay time of the ranging signal. Theranging signal is a signal used for ranging among signalstransmitted/received between the portable device 100 and thecommunication unit 200. The propagation delay time is time fromtransmission to reception of the signal.

Here, the ranging signal is transmitted/received by one of the pluralityof wireless communication sections 210 of the communication unit 200.Hereinafter, the wireless communication section 210 thattransmits/receives the ranging signal is also referred to as a master.The distance R is a distance between the wireless communication section210 serving as the master (more precisely, the antenna 211) and theportable device 100 (more precisely, the antenna 111). In addition, thewireless communication sections 210 other than the wirelesscommunication section 210 that transmits/receives the ranging signal arealso referred to as slaves.

In the ranging process, a plurality of the ranging signals may betransmitted and received between communication unit 200 and the portabledevice 100. Among the plurality of ranging signals, a ranging signaltransmitted from one device to the other device is also referred to as afirst ranging signal. Next, a ranging signal transmitted as a responseto the first ranging signal from the device that has received the firstranging signal to the device that has transmitted the first rangingsignal is also referred to as a second ranging signal. In addition, aranging signal transmitted as a response to the second ranging signalfrom the device that has received the second ranging signal to thedevice that has transmitted the second ranging signal is also referredto as a third ranging signal.

Next, with reference to FIG. 7, an example of a flow of the rangingprocess will be described.

FIG. 7 is a sequence diagram illustrating the example of the flow of theranging process executed in the system 1 according to the presentembodiment. The portable device 100 and the communication unit 200 areinvolved in this sequence. It is assumed that the wireless communicationsection 210A functions as the master in this sequence.

As illustrated in FIG. 7, the portable device 100 first transmits thefirst ranging signal (Step S102). When the wireless communicationsection 210A receives the first ranging signal, the control section 230calculates a CIR of the first ranging signal. Next, the control section230 detects a first incoming wave of the first ranging signal of thewireless communication section 210A on the basis of the calculated CIR(Step S104).

Next, the wireless communication section 210A transmits the secondranging signal in response to the first ranging signal (Step S106). Whenthe second ranging signal is received, the portable device 100calculates a CIR of the second ranging signal. Next, the portable device100 detects a first incoming wave of the second ranging signal on thebasis of the calculated CIR (Step S108).

Next, the portable device 100 transmits the third ranging signal inresponse to the second ranging signal (Step S110). When the wirelesscommunication section 210A receives the third ranging signal, thecontrol section 230 calculates a CIR of the third ranging signal. Next,the control section 230 detects a first incoming wave of the thirdranging signal of the wireless communication section 210A on the basisof the calculated CIR (Step S112).

The portable device 100 measures a time period T₁ from transmission timeof the first ranging signal to reception time of the second rangingsignal, and a time period T₂ from reception time of the second rangingsignal to transmission time of the third ranging signal. Here, thereception time of the second ranging signal is reception time of thefirst incoming wave of the second ranging signal detected in Step S108.Next, the portable device 100 transmits a signal including informationindicating the time period T₁ and the time period T₂ (Step S114). Forexample, such a signal is received by the wireless communication section210A.

The control section 230 measures a time period T₃ from reception time ofthe first ranging signal to transmission time of the second rangingsignal, and a time period T₄ from transmission time of the secondranging signal to reception time of the third ranging signal. Here, thereception time of the first ranging signal is reception time of thefirst incoming wave of the first ranging signal detected in Step S104.In a similar way, the reception time of the third ranging signal isreception time of the first incoming wave of the third ranging signaldetected in Step S112.

Next, the control section 230 estimates the distance R on the basis ofthe time periods T₁, T₂, T₃, and T₄ (Step S116). For example, thecontrol section 230 estimates propagation delay time τ_(m) by using anequation listed below.

$\begin{matrix}{\tau_{m} = \frac{{T_{1} \times T_{4}} - {T_{2} \times T_{3}}}{T_{1} + T_{2} + T_{3} + T_{4}}} & (1)\end{matrix}$

Next, the control section 230 estimates the distance R by multiplyingthe estimated propagation delay time τ_(m) by speed of the signal.

Cause of Reduction in Accuracy of Estimation

The reception times of the ranging signals serving as start or end ofthe time periods T₁, T₂, T₃, and T₄ are reception times of the firstincoming waves of the ranging signals. As described above, the signalsdetected as the first incoming wave are not necessarily the directwaves.

In the case where the combined wave or the delayed wave coming whilebeing delayed behind the direct wave is detected as the first incomingwave, reception time of the first incoming wave varies in comparisonwith the case where the direct wave is detected as the first incomingwave. In this case, an estimation result of the propagation delay timeτ_(m) is changed from a true value (an estimation result obtained in thecase where the direct wave is detected as the first incoming wave). Inaddition, this change deteriorates accuracy of estimating the distance R(hereinafter, also referred to as ranging accuracy).

(2) Angle Estimation

The communication unit 200 performs the angle estimation process. Theangle estimation process is a process of estimating the angles α and βillustrated in FIG. 3. An angle acquisition process includes receptionof an angle estimation signal and calculation of the angles α and β onthe basis of a result of reception of the angle estimation signal. Theangle estimation signal is a signal used for estimating an angle amongsignals transmitted/received between the portable device 100 and thecommunication unit 200. Next, with reference to FIG. 8, an example of aflow of the angle estimation process will be described.

FIG. 8 is a sequence diagram illustrating the example of the flow of theangle estimation process executed in the system 1 according to thepresent embodiment. The portable device 100 and the communication unit200 are involved in this sequence.

As illustrated in FIG. 8, the portable device 100 first transmits theangle estimation signals (Step S202). Next, when the wirelesscommunication sections 210A to 210D receive respective angle estimationsignals, the control section 230 calculates CIRs of the respective angleestimation signals received by the wireless communication sections 210Ato 210D. Next, the control section 230 detects first incoming waves ofthe respective angle estimation signals on the basis of the calculatedCIRs with regard to the wireless communication sections 210A to 210D(Step S204A to Step S204D). Next, the control section 230 detectsrespective phases of the detected first incoming waves with regard tothe wireless communication sections 210A to 210D (Step S206A to StepS206D). Next, the control section 230 estimates the angles α and β onthe basis of the respective phases of the detected first incoming waveswith regard to the wireless communication sections 210A to 210D (StepS208).

Next, details of the process in Step S208 will be described. P_(A)represents the phase of the first incoming wave detected with regard tothe wireless communication section 210A. PB represents the phase of thefirst incoming wave detected with regard to the wireless communicationsection 210B. P_(C) represents the phase of the first incoming wavedetected with regard to the wireless communication section 210C. P_(D)represents the phase of the first incoming wave detected with regard tothe wireless communication section 210D. In this case, antenna arrayphase differences Pd_(AC) and Pd_(BD) in the X axis direction andantenna array phase differences Pd_(BA) and Pd_(DC) in the Y axisdirection are expressed in respective equations listed below.

Pd _(AC)=(P _(A) −P _(C))

Pd _(BD)=(P _(B) −P _(D))

Pd _(DC)=(P _(D) −P _(C))

Pd _(BA)=(P _(B) −P _(A))  (2)

The angles α and β are calculated by using the following equation. Here,λ represents wavelength of a carrier wave of the angle estimationsignal, and d represents a distance between the antennas 211.

α or β=arccos(λ·Pd/(2·π·d))  (3)

Therefore, respective equations listed below represent angles calculatedon the basis of the respective antenna array phase differences.

α_(AC)=arccos(λ·Pd _(AC)/(2·π·d))

α_(BD)=arccos(λ·Pd _(BD)/(2·π·d))

β_(DC)=arccos(λ·Pd _(Dc)/(2·π·d))

β_(BA)=arccos(λ·Pd _(BA)/(2·π·d))  (4)

The control section 230 calculates the angles α and β on the basis ofthe calculated angles α_(AC), α_(BD), β_(DC), and β_(BA). For example,as expressed in the following equations, the control section 230calculates the angles α and β by averaging the angles calculated withregard to the two respective arrays in the X axis direction and the Yaxis direction.

α=(α_(AC)+α_(BD))/2

β=(β_(DC)+β_(BA))/2  (5)

Cause of Reduction in Accuracy of Estimation

As described above, the angles α and β are calculated on the basis ofthe phases of the first incoming waves. As described above, the signalsdetected as the first incoming waves are not necessarily the directwaves.

In other words, sometimes the delayed wave or the combined wave may bedetected as the first incoming wave. Typically, phases of the delayedwave and the combined wave are different from the phase of the directwave. This difference deteriorates accuracy of angle estimation.

Supplement

Note that, the angle estimation signal may be transmitted/receivedduring the angle estimation process, or at any other timings. Forexample, the angle estimation signal may be transmitted/received duringthe ranging process. Specifically, the third ranging signal illustratedin FIG. 7 may be the same as the angle estimation signal illustrated inFIG. 8. In this case, it is possible for the communication unit 200 tocalculate the distance R, the angle α, and the angle β by receiving asingle wireless signal that serves as both the angle estimation signaland the third ranging signal.

(3) Coordinate Estimation

The control section 230 performs a coordinate estimation process. Thecoordinate estimation process is a process of estimatingthree-dimensional coordinates (x, y, z) of the portable device 100illustrated in FIG. 4. As the coordinate estimation process, a firstcalculation method and a second calculation method listed below may beadopted.

First Calculation Method

The first calculation method is a method of calculating the coordinatesx, y, and z on the basis of results of the ranging process and the angleestimation process. In this case, the control section 230 firstcalculates the coordinates x and y by using equations listed below.

x=R·cos α

y=R·cos β  (6)

Here, the distance R, the coordinate x, the coordinate y, and thecoordinate z have a relation represented by an equation listed below.

R=√{square root over (x ² y+z ²)}  (7)

The control section 230 calculates the coordinate z by using theabove-described relation and an equation listed below.

z=√{square root over (R ² −R ²·cos² α−R·cos²β)}  (8)

Second Calculation Method

The second calculation method is a method of calculating the coordinatesx, y, and z while omitting estimation of the angles α and β. First, theabove-listed equations (4), (5), (6), and (7) establish a relationrepresented by equations listed below.

x/R=cos α  (9)

y/R=cos β  (10)

x ² +y ² +z ² =R ²  (11)

d·cos α=λ·(Pd _(AC)/2+Pd _(BD)/2)/(2·π)  (12)

d·cos β=λ·(Pd _(DC)/2+Pd _(BA)/2)/(2·π)  (13)

The equation (12) is rearranged for cos α, and cos α is substituted intothe equation (9). This makes it possible to obtain the coordinate x byusing an equation listed below.

x=R·λ·(Pd _(AC)/2+Pd _(BD)/2)/(2·π·d)  (14)

The equation (13) is rearranged for cos β, and cos β is substituted intothe equation (10). This makes it possible to obtain the coordinate y byusing an equation listed below.

y=R·λ·(Pd _(DC)/2+Pd _(BA)/2)/(2·π·d)  (15)

Next, the equation (14) and the equation (15) are substituted into theequation (11), and the equation (11) is rearranged. This makes itpossible to obtain the coordinate z by using an equation listed below.

z=√{square root over (R ² −x ² −y ²)}  (16)

The process of estimating the coordinates of the portable device 100 inthe local coordinate system has been described above. It is alsopossible to estimate coordinates of the portable device 100 in theglobal coordinate system by combining the coordinates of the portabledevice 100 in the local coordinate system and coordinates of the originof the local coordinate system relative to the global coordinate system.

Cause of Reduction in Accuracy of Estimation

As described above, the coordinates are calculated on the basis of thepropagation delay time and phases. In addition, they are estimated onthe basis of the first incoming waves. Therefore, accuracy of estimatingthe coordinates may deteriorate in a way similar to the ranging processand the angle estimation process.

(4) Estimation of Existence Region

The positional parameter may include a region including the portabledevice 100 among a plurality of predefined regions. For example, in thecase where the region is defined by a distance from the communicationunit 200, the control section 230 estimates the region including theportable device 100 on the basis of the distance R estimated through theranging process. For another example, in the case where the region isdefined by an angle with respect to the communication unit 200, thecontrol section 230 estimates the region including the portable device100 on the basis of the angles α and β estimated through the angleestimation process. For another example, in the case where the region isdefined by the three-dimensional coordinates, the control section 230estimates the region including the portable device 100 on the basis ofthe coordinates (x, y, z) estimated through the coordinate estimationprocess.

Alternatively, in a process specific to the vehicle 202, the controlsection 230 may estimate the region including the portable device 100among the plurality of regions including the vehicle interior and thevehicle exterior of the vehicle 202. This makes it possible to providecourteous service such as providing different serves in the case wherethe user is in the vehicle interior and in the case where the user is inthe vehicle exterior. In addition, the control section 230 may estimatethe region including the portable device 100 among nearby region andfaraway region. The nearby region is a region within a predetermineddistance from the vehicle 202, and the faraway region is thepredetermined distance or more away from the vehicle 202.

(5) Use of Result of Estimating Positional Parameter

For example, a result of estimating the positional parameter may be usedfor authentication of the portable device 100. For example, the controlsection 230 determines that the authentication is successful and unlocka door in the case where the portable device 100 is in a region close tothe communication unit 200 on a driver seat side.

3. Technical Problem

The plurality of wireless communication sections 210 may include both awireless communication section 210 in a line-of-sight (LOS) conditionand a wireless communication section 210 in a non-line-of-sight (NLOS)condition.

The LOS condition means that the antenna 111 of the portable device 100and the antenna 211 of the wireless communication section 210 arevisible from each other. In the case of the LOS condition, a highestreception electric power of the direct wave is obtained. Therefore,there is a high possibility that the receiver succeeds in detecting thedirect wave as the first incoming wave.

The NLOS condition means that the antenna 111 of the portable device 100and the antenna 211 of the wireless communication section 210 are notvisible from each other. In the case of the NLOS condition, receptionelectric power of the direct wave may become lower than the others.Therefore, there is a possibility that the receiver fails in detectingthe direct wave as the first incoming wave.

In the case where the master that transmits/receives the ranging signalin the ranging process is in the NLOS condition, reception electricpower of the direct wave is smaller than noise among ranging signalscoming from the portable device 100. Accordingly, even if detection ofthe direct wave as the first incoming wave is successful, the receptiontime of the first incoming wave may be different due to effects of thenoise. In this case, accuracy of ranging deteriorates.

In addition, in the case where the master that transmits/receive theranging signal in the ranging process is in the NLOS condition,reception electric power of the direct wave becomes lower than the casewhere the master is in the LOS condition, and detection of the directwave as the first incoming wave may end in failure. In this case,accuracy of ranging deteriorates.

Therefore, according to the present embodiment, there is provided thetechnology of selecting, as the master, the wireless communicationsection 210 that is likely to successfully detect the direct wave as thefirst incoming wave in the case of repeating wireless communication forestimating the positional parameter. Such a configuration makes itpossible to improve positional parameter estimation accuracy includingranging accuracy.

4. Technical Features

<4.1. Detection of Specific Element>

The control section 230 detects a specific element on the basis of afirst threshold with regard to each of CIRs respectively obtained fromthe plurality of wireless communication sections 210. The specificelement is one or more of a plurality of elements included in the CIR.Specifically, in the process of detecting the specific element on thebasis of the first threshold, the control section 230 detects one ormore element whose amplitude component included in the CIR value exceedsthe first threshold, as the specific element. The amplitude componentincluded in the CIR value may be amplitude itself or electric powerobtained by squaring the amplitude.

The control section 230 may detect a specific element from a CIR. Inthis case, it is possible to reduce computational load for detecting thespecific element in comparison with the case of detecting the pluralityof specific elements from a CIR.

The first threshold is the above-described first path threshold. In thiscase, the specific element is an element corresponding to the firstincoming wave.

For example, the control section 230 may detect an element havingamplitude or electric power that exceeds the first threshold for thefirst time, as the specific element. The amplitude or electric powerserves as the CIR value. For another example, the control section 230may detect an element having a peak CIR value as the specific element,in a set of elements corresponding to the first incoming wave, whichincludes an element having amplitude or electric power that exceeds thefirst threshold for the first time. The amplitude or electric powerserves as the CIR value.

Time of the specific element serves as the reception time of the firstincoming wave and is used for ranging. Hereinafter, sometimes the timecorresponding to the specific element is also referred to as specificreception time. In addition, the phase of the specific element serves asthe phase of the first incoming wave and is used for angle estimation.Therefore, it is possible to improve the accuracy of estimating apositional parameter by detecting a specific element corresponding tothe direct wave.

<4.2. Reliability Parameter>

The control section 230 calculates a reliability parameter. Thereliability parameter is an indicator indicating whether the detectedfirst incoming wave (that is, specific element) is appropriate for aprocessing target. More specifically, the reliability parameter is anindicator indicating whether it is appropriate to use the detectedspecific element for estimating the positional parameter. When mentionis made of a plurality of the specific elements detected with regard tothe respective wireless communication sections 210, the reliabilityparameter is an indicator indicating whether each of the detectedspecific elements is appropriate for the processing target.

When the specific element is appropriate for the processing target, thespecific element corresponds to the direct wave. On the other hand, whenthe specific element is inappropriate for the processing target, thespecific element does not correspond to the direct wave. In other words,the reliability parameter can be treated as an indicator that indicatessuitability of the detected specific element for an elementcorresponding to the direct wave. In the case where the detectedspecific element corresponds to the delayed wave or the combined wave,that is, in the case where the delayed wave or the combined wave isdetected as the first incoming wave, the accuracy of estimating thepositional parameter deteriorates as described above. Therefore, it ispossible to evaluate the accuracy of estimating the positional parameteron the basis of the reliability parameter.

For example, the reliability parameters are continuous values ordiscrete values. As the reliability parameter has a higher value, thereliability parameter may indicate that the specific element isappropriate for the processing target. In a similar way, as thereliability parameter has a lower value, the reliability parameter mayindicate that the specific element is inappropriate for the processingtarget, and vice versa. Hereinafter, a degree of appropriateness of thespecific element for the processing target may also be referred to asreliability. In addition, high reliability means that the specificelement is appropriate for the processing target, and low reliabilitymeans that the specific element is inappropriate as the processingtarget.

The control section 230 detects the specific elements and calculates thereliability parameters on the basis of the transmission signaltransmitted from the portable device 100 in the positional parameterestimation process and the respective reception signal obtained when theplurality of wireless communication sections 210 receive thetransmission signal. Such a transmission signal may be the rangingsignal or the angle estimation signal. For example, such a transmissionsignal may be a signal that is the third ranging signal illustrated inFIG. 7 and that also serves as the angle estimation signal.

Next, examples of the reliability parameter will be described.

First Reliability Parameter

The reliability parameter may include a first reliability parameter thatis a difference between time corresponding to an element having a CIRvalue that exceeds the first path threshold for the first time in a CIRand time corresponding to an element having the maximum CIR value in theCIR.

Hereinafter, the time corresponding to the element having the CIR valuethat exceeds the first path threshold for the first time in the CIR willbe referred to as a first path determination position. For example, thefirst path determination position is time corresponding to an elementhaving the amplitude or electric power that exceeds the first paththreshold for the first time. The amplitude or electric power serves asthe CIR value in the CIR. Note that, the first path determinationposition is synonymous with the reception time of the first incomingwave described above.

Hereinafter, the time corresponding to the element having the maximumCIR value in the CIR will also be referred to as a peak path position.For example, the peak path position is time corresponding to an elementhaving the maximum amplitude or electric power. The amplitude orelectric power serves as the CIR value in the CIR.

Hereinafter, with reference to FIG. 9 and FIG. 10, first pathdetermination positions and peak path positions in the LOS condition andthe NLOS condition will be described.

FIG. 9 is a graph illustrating an example of a CIR with regard to thewireless communication section 210 in the LOS condition. FIG. 10 is agraph illustrating an example of a CIR with regard to the wirelesscommunication section 210 in the NLOS condition. The graph includes ahorizontal axis representing delay time. The graph includes a verticalaxis representing absolute values of CIR values (such as electricpower). The CIRs illustrated in FIG. 9 and FIG. 10 include a set 21 ofelements corresponding to the direct wave, and a set 22 of elementscorresponding to the delayed wave.

In the example illustrated in FIG. 9, the set 21 includes a first pathdetermination position T_(FP) having a CIR value that exceeds the firstpath threshold TH_(FP) for the first time. In other words, the set 21corresponds to the first incoming wave. In addition, the set 21 includesa peak path position T_(PP).

In the example illustrated in FIG. 10, the set 21 includes a first pathdetermination position T_(FP) having a CIR value that exceeds the firstpath threshold TH_(FP) for the first time. In other words, the set 21corresponds to the first incoming wave. In addition, the set 22 includesa peak path position T_(PP).

As illustrated in FIG. 9, in the case of the LOS condition, a peak ofthe whole CIR appears in the set 21 corresponding to the direct wave. Onthe other hand, as illustrated in FIG. 10, in the case of the NLOScondition, a peak of the whole CIR appears in the set 22 correspondingto the delayed wave. This is because there is an obstacle in the firstpath in the case of the NLOS condition. In particular, if a human bodyis interposed in the first path, the direct wave drastically attenuateswhen the direct wave passes through the human body. Therefore, adifference Pt between the first path determination position T_(FP) andthe peak path position T_(PP) in the LOS condition illustrated in FIG. 9is smaller than a difference Pt between the first path determinationposition T_(FP) and the peak path position T_(PP) in the NLOS conditionillustrated in FIG. 10. Therefore, the control section 230 determinesthat reliability gets higher as the difference Pt decreases. On theother hand, the control section 230 determines that reliability getslower as the difference Pt increases. As described above, it is possibleto evaluate the reliability from a viewpoint of the difference Pt.

Therefore, among the respective reliability parameters calculated withrespect to the plurality of wireless communication sections 210, thecontrol section 230 may treat a reliability parameter having a smallestdifference Pt or a difference Pt that is s smaller than a secondthreshold, as the optimum parameter. In other words, the control section230 may select a wireless communication section 210 having the smallestdifference Pt or a difference Pt that is smaller than the secondthreshold as the master from among the wireless communication sections210. Note that, a threshold TH_(XL) (to be described later) serves as anexample of the second threshold. Such a configuration makes it possibleto select a wireless communication section 210 that is most likely to bein the LOS condition, as the master. Therefore, it is possible toimprove the accuracy of ranging rather than the case of selecting awireless communication section 210 that is in the NLOS condition as themaster.

For example, the control section 230 may calculate an NLOS rate R_(NLOS)on the basis of the difference Pt. Next, the master may be selected onthe basis of the NLOS rate R_(NLOS). The NLOS rate R_(NLOS) is aparameter indicating a probability that the wireless communicationsection 210 is in the NLOS condition. The NLOS rate R_(NLOS) is a valuethat is 0 or more and 1 or less. For example, when the NLOS rateR_(NLOS) is closer to 1, this means that the wireless communicationsection 210 is in the NLOS condition. On the other hand, when the NLOSrate R_(NLOS) is closer to zero, this means that the wirelesscommunication section 210 is in the LOS condition. In this case, thecontrol section 230 selects the wireless communication section 210having a minimum NLOS rate R_(NLOS) as the master. This makes itpossible to select the wireless communication section 210 that is mostlikely to be in the LOS condition, as the master.

Next, an example of a method of calculating the NLOS rate R_(NLOS) willbe described. For example, the control section 230 may calculate theNLOS rate R_(NLOS) by using two thresholds TH_(XL) and TH_(XH). Notethat, it is assumed that TH_(XL)<TH_(XH) is satisfied.

The control section 230 determines that the NLOS rate R_(NLOS) is zeroin the case where the difference Pt 5 the threshold TH_(XL). This isbecause, in the case of the LOS condition as illustrated in FIG. 9, theset 21 corresponding to the direct wave includes both the first pathdetermination position T_(FP) and the peak path position T_(PP). Forexample, in the case where the set 21 corresponding to the direct waveincludes both the first path determination position T_(FP) and the peakpath position T_(PP), the threshold TH_(XL) is set to any value thatsatisfies a relation: the difference Pt≤the threshold TH_(XL).

Here, it is assumed that only the direct wave is received as the firstincoming wave. It is unclear which element has the CIR value thatexceeds the first path value among the elements corresponding to thefirst incoming wave in the CIR. Even if the element having the CIR valuethat exceeds the first path value is an earliest element in the timedirection among the elements corresponding to the first incoming wave inthe CIR, the difference Pt is a half of the width of the first incomingwave. As described above, in the case where only the direct wave isreceived as the first incoming wave, the first incoming wave of the CIRhas an ideal width. Therefore, an example of a condition regarding thethreshold TH_(XL) is a condition that the threshold TH_(XL)≤a half ofthe ideal width obtained in the case where only the direct wave isdetected as the first incoming wave is satisfied.

The control section 230 determines that the NLOS rate R_(NLOS) is 1 inthe case where the difference Pt≥the threshold TH_(XH). This is because,in the case of the NLOS condition as illustrated in FIG. 10, the set 21corresponding to the direct wave includes the first path determinationposition T_(FP) and the set 22 corresponding to the delayed wave mayinclude the peak path position T_(PP). For example, in the case wherethe first path determination position T_(FP) and the peak path positionT_(PP) appear in the respective sets that are different from each other,the threshold TH_(XH) is set to any value that satisfies a relation: thedifference Pt≥the threshold TH_(XH).

In the case where the threshold TH_(XL)<the difference Pt<the thresholdTH_(XH), the control section 230 calculates a smaller NLOS rate R_(NLOS)as the difference Pt decreases. For example, the control section 230 maycalculates the NLOS rate R_(NLOS) by using an equation listed below.

$\begin{matrix}{R_{NLOS} = \frac{{Pt} - {TH_{XL}}}{{TH_{X\; H}} - {TH_{XL}}}} & (17)\end{matrix}$

Second Reliability Parameter

The reliability parameter may include a second reliability parameter.The second reliability parameter is a result of comparing amplitude orelectric power corresponding to the element having the CIR value thatexceeds the first threshold for the first time in the CIR with amplitudeor electric power corresponding to the element having the maximum CIRvalue in the CIR.

Hereinafter, the amplitude corresponding to the element having the CIRvalue that exceeds the first path threshold for the first time in theCIR will be referred to as first path amplitude. For example, the firstpath amplitude may be amplitude of an element having the amplitude orelectric power that exceeds the first path threshold for the first time.The amplitude or electric power serves as the CIR value in the CIR. Foranother example, the first path amplitude may be amplitude of a first orsubsequent element after the element having the amplitude or electricpower that exceeds the first path threshold for the first time. Theamplitude or electric power serves as the CIR value in the CIR. Forexample, the first path amplitude may be amplitude of an element havinga peak amplitude or electric power for the first time after the elementhaving the amplitude or electric power that exceeds the first paththreshold for the first time. The amplitude or electric power serves asthe CIR value in the CIR. For another example, the first path amplitudemay be a CIR area from a first element having the amplitude or electricpower that exceeds the first path threshold for the first time to asecond element that is a first or subsequent element after the firstelement. The amplitude or electric power serves as the CIR value in theCIR. Note that, the CIR area described herein is the integral of theamplitudes serving as the CIR values from the first element to thesecond element.

Hereinafter, the electric power corresponding to the element having theCIR value that exceeds the first path threshold for the first time inthe CIR will be referred to as first path electric power. For example,the first path electric power may be electric power of an element havingthe amplitude or electric power that exceeds the first path thresholdfor the first time. The amplitude or electric power serves as the CIRvalue in the CIR. For another example, the first path electric power maybe electric power of a first or subsequent element after the elementhaving the amplitude or electric power that exceeds the first paththreshold for the first time. The amplitude or electric power serves asthe CIR value in the CIR. For example, the first path electric power maybe electric power of an element having a peak amplitude or electricpower for the first time after the element having the amplitude orelectric power that exceeds the first path threshold for the first time.The amplitude or electric power serves as the CIR value in the CIR. Foranother example, the first path electric power may be a CIR area from afirst element having the amplitude or electric power that exceeds thefirst path threshold for the first time to a second element that is afirst or subsequent element after the first element. The amplitude orelectric power serves as the CIR value in the CIR. Note that, the CIRarea described herein is the integral of the electric powers serving asthe CIR values from the first element to the second element.

Hereinafter, the amplitude corresponding to the element having themaximum CIR value in the CIR will be referred to as a peak pathamplitude. For example, the peak path amplitude may be amplitude of anelement having the maximum amplitude or electric power. The amplitude orelectric power serves as the CIR value in the CIR. For another example,the peak path amplitude may be amplitude of a first or subsequentelement after the element having the maximum amplitude or electricpower. The amplitude or electric power serves as the CIR value in theCIR. For another example, the peak path amplitude may be a CIR area withregard to a plurality of elements that are subsequent to the elementhaving the maximum amplitude or electric power in the time direction.The amplitude or electric power serves as the CIR value in the CIR. Notethat, the CIR area described herein is the integral of the amplitudesserving as the CIR values from a third element to a fourth element. Thethird element is a leading element and the fourth element is an endelement, among the plurality of elements that are subsequent to theelement having the maximum amplitude or electric power in the timedirection. The third element is the element having the maximum amplitudeor electric power, or a first or more preceding element before such anelement. The amplitude or electric power serves as the CIR value in theCIR. The fourth element is the element having the maximum amplitude orelectric power, or a first or subsequent element after such an element.The amplitude or electric power serves as the CIR value in the CIR. Forexample, the CIR area may be a CIR area with regard to 2X+1 number ofelements that include the element having the maximum amplitude orelectric power and X number of elements that are disposed before orafter such an element. The amplitude or electric power serves as the CIRvalue in the CIR.

Hereinafter, the electric power corresponding to the element having themaximum CIR value in the CIR will be referred to as a peak path electricpower. For example, the peak path electric power may be electric powerof an element having the maximum amplitude or electric power. Theamplitude or electric power serves as the CIR value in the CIR. Foranother example, the peak path electric power may be electric power of afirst or subsequent element after the element having the maximumamplitude or electric power. The amplitude or electric power serves asthe CIR value in the CIR. For another example, the peak path electricpower may be a CIR area with regard to a plurality of elements that aresubsequent to the element having the maximum amplitude or electric powerin the time direction. The amplitude or electric power serves as the CIRvalue in the CIR. Note that, the CIR area described herein is theintegral of the electric powers serving as the CIR values from the thirdelement to the fourth element. The third element is a leading elementand the fourth element is an end element, among the plurality ofelements that are subsequent to the element having the maximum amplitudeor electric power in the time direction.

Hereinafter, with reference to FIG. 11 and FIG. 12, the first pathelectric power and peak path electric power in the LOS condition and theNLOS condition will be described. FIG. 11 is a graph illustrating anexample of a CIR with regard to the wireless communication section 210in the LOS condition. FIG. 12 is a graph illustrating an example of aCIR with regard to the wireless communication section 210 in the NLOScondition. The graph includes a horizontal axis representing delay time.The graph includes a vertical axis representing absolute values of CIRvalues (such as electric power). The CIRs illustrated in FIG. 11 andFIG. 12 include a set 21 of elements corresponding to the direct wave,and a set 22 of elements corresponding to the delayed wave.

In the example illustrated in FIG. 11, the set 21 includes an elementhaving a CIR value that exceeds the first path threshold TH_(FP) for thefirst time. In other words, the set 21 corresponds to the first incomingwave. As illustrated in FIG. 11, in the case of the LOS condition, apeak of the whole CIR appears in the set 21 corresponding to the directwave. Therefore, in the case of the LOS condition, the first pathelectric power P_(FP) is typically identical to the peak path electricpower P_(PP). Note that, in the case where the first path electric powerP_(FP) and the peak path electric power PPP are calculated in differentways, the first path electric power P_(FP) is substantially identical tothe peak path electric power P_(PP),

In the example illustrated in FIG. 12, the set 21 includes an elementhaving a CIR value that exceeds the first path threshold TH_(FP) for thefirst time. In other words, the set 21 corresponds to the first incomingwave. As illustrated in FIG. 12, in the case of the NLOS condition, apeak of the whole CIR appears in the set 22 corresponding to the delayedwave. This is because there is an obstacle in the first path in the caseof the NLOS condition. In particular, if a human body is interposed inthe first path, the direct wave drastically attenuates when the directwave passes through the human body. Therefore, in the case of the NLOScondition, sometimes the first path electric power P_(FP) may besignificantly smaller than the peak path electric power PPP.

As described above, the control section 230 determines that reliabilitygets higher as the first path electric power gets closer to the peakpath electric power. On the other hand, the control section 230determines that reliability gets lower as the first path electric powergets farther away from the peak path electric power. As described above,it is possible to evaluate the reliability from a viewpoint of arelative relation between the first path electric power P_(FP) and thepeak path electric power PPP. Note that, the first path electric powerand the peak path electric power have been described above withreference to FIG. 11 and FIG. 12, and the same applies to the first pathamplitude and the peak path amplitude.

Therefore, among the respective reliability parameters calculated withregard to the plurality of wireless communication sections 210, thecontrol section 230 treats a reliability parameter indicating thatamplitude or electric power corresponding to the element having the CIRvalue that exceeds the first threshold for the first time in the CIR isclosest to amplitude or electric power corresponding to the elementhaving the maximum CIR value in the CIR, as the optimum parameter. Forexample, from among the plurality of wireless communication sections210, the control section 230 selects a wireless communication section210 having the first path amplitude that is closest to the peak pathamplitude, as the master. For another example, from among the pluralityof wireless communication sections 210, the control section 230 selectsa wireless communication section 210 having the first path electricpower that is closest to the peak path electric power, as the master.

Specifically, the control section 230 may calculate at least one of anamplitude ratio and an electric power ratio as the second reliabilityparameter. The amplitude ratio is a parameter indicating a result ofcomparing the first path amplitude with the peak path amplitude. Theelectric power ratio is a parameter indicating a result of comparing thefirst path electric power with the peak path electric power.

Next, an example of methods of calculating the amplitude ratio and theelectric power ratio will be described. For example, the amplitude ratioand the electric power ratio may be calculated by using equations listedbelow.

Amplitude Ratio=First path Amplitude/Peak Path Amplitude

Amplitude Ratio=First path Amplitude−Peak Path Amplitude

Electric Power Ratio=First path Electric Power/Peak Path Electric Power

Electric Power Ratio=First path Electric Power−Peak Path Electric Power

From among the plurality of wireless communication sections 210, thecontrol section 230 may select a wireless communication section 210having a highest amplitude ratio, that is, the wireless communicationsection 210 having the first path amplitude that is closest to the peakpath amplitude, as the master. Alternatively, from among the pluralityof wireless communication sections 210, the control section 230 mayselect a wireless communication section 210 having a highest electricpower ratio, that is, the wireless communication section 210 having thefirst path electric power that is closest to the peak path electricpower, as the master. Such a configuration makes it possible to select awireless communication section 210 that is most likely to be in the LOScondition, as the master. Therefore, it is possible to improve theaccuracy of ranging rather than the case of selecting a wirelesscommunication section 210 that is in the NLOS condition as the master.

Third Reliability Parameter

The reliability parameter may include a third reliability parameter thatis time corresponding to an element having a CIR value that exceeds thefirst path threshold for the first time in a CIR. In other words, thereliability parameter may include the first path determination position.

Hereinafter, with reference to FIG. 13, first path determinationpositions with regard to the plurality of wireless communicationsections 210 will be described. FIG. 13 is graphs illustrating anexample of CIRs with regard to the plurality of wireless communicationsections 210. A CIR 20A illustrated in FIG. 13 is a graph illustratingan example of a CIR with regard to the wireless communication section210A in the LOS condition. A CIR 20B illustrated in FIG. 13 is a graphillustrating an example of a CIR with regard to the wirelesscommunication section 210B in the NLOS condition. Each graph includes ahorizontal axis representing delay time. It is assumed that a time axisof the CIR 20A is synchronous with a time axis of the CIR 20B. The graphincludes a vertical axis representing absolute values of CIR values(such as electric power).

The CIR 20A includes a set 21A of elements corresponding to the directwave, and a set 22A of elements corresponding to the delayed wave. Withregard to the CIR 20A, the set 21A includes a first path determinationposition T_(FP-A) having a CIR value that exceeds the first paththreshold TH_(FP) for the first time. In other words, the set 21Acorresponds to the first incoming wave with regard to the CIR 20A.

The CIR 20B includes a set 21B of elements corresponding to the directwave, and a set 22B of elements corresponding to the delayed wave. Withregard to the CIR 20B, the set 22B includes a first path determinationposition T_(FP-B) having a CIR value that exceeds the first paththreshold TH_(FP) for the first time. In other words, the set 22Bcorresponds to the first incoming wave with regard to the CIR 20B.

As illustrated in FIG. 13, the first path determination positionT_(FP-A) appears in the set 21A corresponding to the direct wave withregard to the wireless communication section 210A in the LOS condition.On the other hand, the first path determination position T_(FP-B) mayappear not in the set 21B corresponding to the direct wave, but in theset 22B corresponding to the delayed wave with regard to the wirelesscommunication section 210B in the NLOS condition. In other words, it canbe said that an early first path determination position indicates a highpossibility of successfully detecting the direct wave as the firstincoming wave. On the other hand, it can be said that a late first pathdetermination position indicates a high possibility that detection ofthe direct wave as the first incoming wave ends in failure. Therefore,the control section 230 determines that reliability gets higher as anearlier first path determination position is obtained. On the otherhand, the control section 230 determines that reliability gets lower asa later first path determination position is obtained. As describedabove, it is possible to evaluate the reliability from a viewpoint ofthe first path determination position.

Therefore, among the respective reliability parameters calculated withrespect to the plurality of wireless communication sections 210, thecontrol section 230 treats a reliability parameter indicating anearliest first path determination position, as the optimum parameter. Inother words, from among the plurality of wireless communication sections210, the control section 230 selects a wireless communication section210 having the earliest first path determination position, as themaster. Such a configuration makes it possible to select a wirelesscommunication section 210 that is most likely to have successfullydetected the direct wave as the first incoming wave, as the master.Therefore, it is possible to improve the accuracy of ranging rather thanthe case of selecting a wireless communication section 210 that hasdetected a wave other than the direct wave as the first incoming wave,as the master.

In the case of repeatedly performing the ranging process, the controlsection 230 may control whether to reselect the master on the basis of adifference between the first path determination position of the slaveand the first path determination position of the master in a previousranging process. For example, the control section 230 calculates adifference Tdif between the first path determination position of themaster and an earliest first path determination position correspondingto one of the plurality of wireless communication sections 210, by usingan equation listed below.

Tdif=T _(FP-m)−min(T _(FP-m) ,T _(FP-s1) ,T _(FP-s2) ,T _(FP-s3))  (18)

Here, T_(FP-m) is the first path determination position of the master.T_(FP-s1), T_(FP-s2), and T_(FP-s3) are first path determinationpositions of respective slaves.

Next, the control section 230 may reselect a master in the case wherethe difference Tdif exceeds a predetermined threshold. When thedifference Tdif exceeds the predetermined threshold, this means that thefirst path determination position of the master is a predeterminedthreshold or more behind the earliest first path determination positioncorresponding to one of the plurality of wireless communication sections210. This means that the master is likely to have failed in detection ofthe direct wave as the first incoming wave but the slave is likely tohave successfully detected the direct wave as the first incoming wave.Therefore, it is possible to improve the accuracy of ranging by reselectthe slave having the earliest first path determination position as themaster from among the plurality of wireless communication sections 210.Note that, for example, the predetermined threshold is a half of anideal width obtained in the case where only the direct wave is detectedas the first incoming wave.

Fourth Reliability Parameter

The reliability parameter may include a fourth reliability parameterderived from correlation between CIR waveforms of the wirelesscommunication sections 210 in a pair.

Hereinafter, with reference to FIG. 14, CIR waveforms with regard to theplurality of wireless communication sections 210 will be described. FIG.14 is graphs illustrating an example of CIRs with regard to theplurality of wireless communication sections 210. A CIR 20A illustratedin FIG. 14 is a graph illustrating an example of a CIR with regard tothe wireless communication section 210A. A CIR 20B illustrated in FIG.14 is a graph illustrating an example of a CIR with regard to thewireless communication section 210B. Each graph includes a horizontalaxis representing delay time. It is assumed that the wirelesscommunication section 210A and the wireless communication section 210 Bare in synchronization with each other. In other words, it is assumedthat a time axis of the CIR 20A is synchronous with a time axis of theCIR 20B. The graph includes a vertical axis representing absolute valuesof CIR values (such as amplitude or electric power).

The CIR 20A includes a set 23A of elements corresponding to the combinedwave received in a state where the direct wave is combined with thedelayed wave having a different phase from the direct wave. The CIRwaveform of the set 23A has two peaks because two waves having differentphases are combined. With regard to the CIR 20A, the set 23A includes anelement having a CIR value that exceeds the first path threshold TH_(FP)for the first time. In other words, the set 23A corresponds to the firstincoming wave with regard to the CIR 20A.

On the other hand, the CIR 20B includes a set 23B of elementscorresponding to the combined wave received in a state where the directwave is combined with the delayed wave having a same phase as the directwave. The CIR waveform of the set 23 has a single large peak because twowaves having the same phase are combined. With regard to the CIR 20B,the set 23B includes an element having a CIR value that exceeds thefirst path threshold TH_(FP) for the first time. In other words, the set23B corresponds to the first incoming wave with regard to the CIR 20B.

In the case where the plurality of wireless communication sections 210receive signals in the state where the direct wave is combined with thedelayed wave, the wireless communication sections 210 may have differentrelations of phases of the direct wave and the delayed wave even if adistance between the wireless communication sections 210 is short. As aresult, different CIR waveforms are obtained as illustrated in the CIR20A and CIR 20B. In other words, the different CIR waveforms between thewireless communication sections 210 in a pair mean that a combined waveis received by at least one of the wireless communication sections 210in the pair. In the case where the combined wave is detected as thefirst incoming wave, the accuracy of angle estimation in the angleestimation process deteriorates as described above. In addition, in thecase where the combined wave is detected as the first incoming wave, theaccuracy of ranging in the ranging process deteriorates as describedabove.

Accordingly, the fourth reliability parameter may be a correlationcoefficient between a CIR obtained on the basis of a reception signalreceived by a first wireless communication section 210 among theplurality of wireless communication sections 210, and a CIR obtained onthe basis of a reception signal received by a second wirelesscommunication section 210 that is different from the first wirelesscommunication section 210 among the plurality of wireless communicationsections 210. In other words, the fourth reliability parameter may be acorrelation coefficient between a waveform of the entire CIR calculatedwith regard to the first wireless communication section 210 and awaveform of the entire CIR calculated with regard to the second wirelesscommunication section 210. In addition, the control section 230determines that reliability gets higher as the correlation coefficientincreases. On the other hand, the control section 230 determines thatreliability gets lower as the correlation coefficient decreases. Such aconfiguration makes it possible to evaluate reliability from a viewpointof correlation between CIR waveforms.

Here, a phase of the first incoming wave is used for the angleestimation process. Therefore, the reliability parameter may be derivedfrom correlation between CIR waveforms close to the first incoming wave.

Accordingly, the fourth reliability parameter may be a correlationcoefficient between chronological change in CIR value of a portionincluding the element having the CIR value that exceeds the first paththreshold for the first time in a CIR obtained on the basis of areception signal received by the first wireless communication section210 among the plurality of wireless communication sections 210, andchronological change in CIR value of a portion including the elementhaving the CIR value that exceeds the first path threshold for the firsttime in a CIR obtained on the basis of a reception signal received bythe second wireless communication section 210 that is different from thefirst wireless communication section 210 among the plurality of wirelesscommunication sections 210. Here, the portion means a set including theelement having the CIR value that exceeds the first path threshold forthe first time, and one or more elements that exist before and/or aftersuch an element in the time axis direction. In other words, the fourthreliability parameter may be a correlation coefficient between awaveform obtained in a vicinity of a first path determination positionin the CIR calculated with regard to the first wireless communicationsection 210, and a waveform obtained in a vicinity of a first pathdetermination position in the CIR calculated with regard to the secondwireless communication section 210. In addition, the control section 230determines that reliability gets higher as the correlation coefficientincreases. On the other hand, the control section 230 determines thatreliability gets lower as the correlation coefficient decreases. Such aconfiguration makes it possible to evaluate reliability from a viewpointof correlation between CIR waveforms obtained in the vicinity of thefirst incoming wave. In addition, such a configuration makes it possibleto reduce an amount of calculation in comparison with the case ofcorrelating waveforms of the whole CIRs.

The control section 230 treats a highest correlation coefficient as theoptimum parameter among correlation coefficients calculated with regardto the plurality of the pairs of the wireless communication sections210. Specifically, the control section 230 selects one of two wirelesscommunication sections 210 included in the pair from which the highestcorrelation coefficient is calculated as the master, among correlationcoefficients calculated with regard to the plurality of the pairs of thewireless communication sections 210. Such a configuration makes itpossible to select a wireless communication section 210 that is unlikelyto have detected the combined wave as the first incoming wave, that is,the wireless communication section 210 that is most likely to havesuccessfully detected the direct wave as the first incoming wave, as themaster. This makes it possible to improve the accuracy of ranging ratherthan the case of detecting the combined wave as the first incoming wave.

Note that, the correlation coefficient may be the Pearson correlationcoefficient, for example.

The CIR may include amplitude or electric power, which is a CIR value,as an element obtained at each time. In this case, the control section230 calculates a correlation coefficient by correlating respectiveamplitudes or electric powers obtained at corresponding times, which areincluded in the two CIRs. Note that, the corresponding times indicate asame time in an environment where the time axes of the two CIRs aresynchronous with each other.

The CIR may include a complex number, which is a CIR value, as theelement obtained at each time. In this case, the control section 230calculates a correlation coefficient by correlating respective complexnumbers obtained at corresponding times, which are included in the twoCIRs. The complex number includes a phase component in addition to anamplitude component. Therefore, it is possible to calculate a moreaccurate correlation coefficient than the case of calculating acorrelation coefficient on the basis of amplitude or electric power.

Fifth Reliability Parameter

The fifth reliability parameter is an indicator that indicates whetherthe first incoming wave itself is appropriate for the detection target.In other words, the fifth reliability parameter is an indicator thatindicates whether the specific element itself is appropriate for thedetection target. Higher reliability is obtained as the first incomingwave is more appropriate for the detection target, and lower reliabilityis obtained as the first incoming wave is more inappropriate for thedetection target.

Specifically, the fifth reliability parameter may be an indicator thatindicates magnitude of noise. In this case, the fifth reliabilityparameter is calculated on the basis of at least any of asignal-to-noise ratio (SNR) and electric power of the first incomingwave. In the case where the electric power is high, effects of the noiseare small. Therefore, the fifth reliability parameter indicating thatthe first incoming wave is appropriate for the detection target iscalculated. On the other hand, in the case where the electric power islow, effects of the noise are large. Therefore, the fifth reliabilityparameter indicating that the first incoming wave is inappropriate forthe detection target is calculated. In the case where the SNR is high,the effects of the noise are small. Therefore, the fifth reliabilityparameter indicating that the first incoming wave is appropriate for thedetection target is calculated. On the other hand, in the case where theSNR is low, effects of the noise are large. Therefore, the fifthreliability parameter indicating that the first incoming wave isinappropriate for the detection target is calculated.

By using the fifth reliability parameter, it is possible to evaluatereliability on the basis of whether the first incoming wave itself isappropriate for the detection target.

Sixth Reliability Parameter

The sixth reliability parameter is an indicator that indicatesunsuitability of a combined wave for the first incoming wave. In otherwords, the sixth reliability parameter is an indicator that indicatesunsuitability of the combined wave for the specific element. Higherreliability is obtained as the unsuitability of the combined wave forthe first incoming wave gets higher, and lower reliability is obtainedas the suitability of the combined wave for the first incoming wave getslower.

Specifically, the sixth reliability parameter is calculated on the basisof at least any of width of the first incoming wave in the timedirection and a state of the phase of the first incoming wave.

First, with reference to FIG. 15, calculation of the sixth reliabilityparameter based on the width of the first incoming wave in the timedirection will be described. Here, the width of the first incoming wavein the time direction may be width of a set of elements corresponding tothe first incoming wave in the time direction, with regard to the CIR.

FIG. 15 is diagrams for describing examples of the reliability parameteraccording to the present embodiment. In the case where the direct waveis independently received as illustrated in the top of FIG. 15, width Wof a set 21 of elements corresponding to the direct wave in the CIRserves as an ideal width obtained when only the direct wave is detectedas the first incoming wave. Here, the width W is width of a set ofelements corresponding to a single pulse in the time direction. Forexample, the width W is width between a zero-crossing and anotherzero-crossing. For another example, the width W is width betweenintersections of a standard and varied CIR values. On the other hand,when the wireless communication sections 210 receive signals in a statewhere the plurality of pulses coming through different paths arecombined due to effects of multipath, the width W of a set of elementscorresponding to the combined wave in the CIR may be different from theideal width obtained when only the direct wave is detected as the firstincoming wave. For example, when a direct wave and a delayed wave arereceived in such a manner that the delayed wave having a same phase asthe direct wave is combined with the direct wave as illustrated in thebottom of FIG. 15, the set 21 of elements corresponding to the directwave and the set 22 of elements corresponding to the delayed wave areadded in a state where they are shifted in the time direction.Therefore, the set 23 of elements corresponding to the combined wave inthe CIR has a wide width W. On the other hand, when a direct wave and adelayed wave are received in such a manner that the delayed wave havingan opposite phase from the direct wave is combined with the direct wave,the direct wave and the delayed wave annihilate each other. Therefore, aset of elements corresponding to the combined wave in the CIR has anarrow width W.

As described above, the sixth reliability parameter is calculated insuch a manner that the sixth reliability parameter indicates that theunsuitability of the combined wave for the first incoming wave getshigher as a difference between the width of the first incoming wave andthe ideal width obtained when only the direct wave is detected as thefirst incoming wave gets smaller. On the other hand, the sixthreliability parameter is calculated in such a manner that the sixthreliability parameter indicates that the unsuitability of the combinedwave for the first incoming wave gets lower as the difference betweenthe width of the first incoming wave and the ideal width obtained whenonly the direct wave is detected as the first incoming wave gets larger.

Next, with reference to FIG. 16, calculation of the sixth reliabilityparameter based on a state of phase of the first incoming wave will bedescribed. Here, the state of the phase of the first incoming wave maybe a degree of difference in phase between elements corresponding to thefirst incoming wave with regard to the received wireless signal.Alternatively, the state of the phase of the first incoming wave may bea degree of difference in phase between elements corresponding to thefirst incoming wave with regard to the CIR.

FIG. 16 is diagrams for describing examples of the reliability parameteraccording to the present embodiment. In the case where the direct waveis independently received as illustrated in the top of FIG. 16,respective phases θ of a plurality of elements belonging to the set 21of elements corresponding to the direct wave in the CIR are a same orsubstantially same phases (that is, θ1≈θ2≈θ3). Note that, the phase isan angle between IQ components of a CIR and an I axis on an IQ plane.This is because distances of paths of direct waves are the same withregard to the respective elements. On the other hand, in the case wherethe combined wave is received as illustrated in the bottom of FIG. 16,respective phases θ of a plurality of elements belonging to the set 23of elements corresponding to the combined wave in the CIR are differentphases (that is, θ1≠θ2≠θ3). This is because pulses having differentdistances between the transmitter and the receivers, that is, the pulseshaving different phases are combined. As described above, the sixthreliability parameter is calculated in such a manner that the sixthreliability parameter indicates that the unsuitability of the combinedwave for the first incoming wave gets higher as the difference betweenthe phases of elements corresponding to the first incoming wave getssmaller. On the other hand, the sixth reliability parameter is alsocalculated in such a manner that the sixth reliability parameterindicates that the unsuitability of the combined wave for the firstincoming wave gets lower as the difference between the phases of theelements corresponding to the first incoming wave gets larger.

By using the sixth reliability parameter, it is possible to evaluate thereliability on the basis of the unsuitability of the combined wave forthe first incoming wave.

Seventh Reliability Parameter

The reliability parameter may include a seventh reliability parameterthat is a difference between time corresponding to a first element andtime corresponding to a second element of the CIR. The first element hasa peak CIR value for the first time after the specific element, and thesecond element has a peak CIR value for the second time after thespecific element. Details of the seventh reliability parameter will bedescribed with reference to FIG. 17 and FIG. 18.

FIG. 17 and FIG. 18 are graphs illustrating examples of the CIRs. Thegraph includes a horizontal axis representing delay time. The graphincludes a vertical axis representing absolute values of CIR values(such as electric power or amplitude).

The CIR illustrated in FIG. 17 include a set 21 of elementscorresponding to the direct wave, and a set 22 of elements correspondingto the delayed wave. The set 21 includes a specific element SP_(FP) thatis an element whose CIR value exceeds the first path threshold TH_(FP)for the first time. In other words, the set 21 corresponds to the firstincoming wave. The set 21 includes a first element SP_(P1) having a peakCIR value for the first time after the specific element SP_(FP). On theother hand, the set 22 includes a second element SP_(P2) having a peakCIR value for the second time after the specific element SP_(FP).

The CIR illustrated in FIG. 18 includes a set 23 of elementscorresponding to the combined wave received in a state where the directwave is combined with the delayed wave having a different phase from thedirect wave. The CIR waveform of the set 23 has two peaks because twowaves having different phases are combined. The set 23 includes aspecific element SP_(FP) that is an element whose CIR value exceeds thefirst path threshold TH_(FP) for the first time. In other words, the set23 corresponds to the first incoming wave. The set 23 includes a firstelement SP_(P1) having a peak CIR value for the first time after thespecific element SP_(FP). The set 23 includes a second element SP_(P2)having a peak CIR value for the second time after the specific elementSP_(FP).

In the case where the direct wave is detected as the first incomingwave, the first incoming wave has a CIR waveform with a single peak asillustrated in FIG. 17. On the other hand, in the case where thecombined wave is detected as the first incoming wave, the first incomingwave may have a CIR waveform with multiple peaks as illustrated in FIG.18. In addition, it is possible to determine whether the first incomingwave has the CIR waveform with the single peak or the multiple peaks onthe basis of a difference T_(P1-P2) between the time T_(P1)corresponding to the first element SP_(P1) and the time T_(P2)corresponding to the second element SP_(P2). This is because a largedifference T_(P1-P2) is obtained in the case where the first incomingwave has the CIR waveform with the single peak. In addition, a smallerdifference T_(P1-P2) is obtained in the case where the first incomingwave has the CIR waveform with the multiple peaks.

In the case where the combined wave is detected as the first incomingwave, accuracy of estimating the positional parameter deteriorates incomparison with the case where the direct wave is detected as the firstincoming wave. Therefore, it can be said that the larger differenceT_(P1-P2) means higher reliability. As described above, it is possibleto evaluate reliability by using the difference T_(P1-P2). Thedifference T_(P1-P2) is the seventh reliability parameter.

Eighth Reliability Parameter

The eighth reliability parameter is an indicator that indicatessuitability of a direct wave for the first incoming wave. In otherwords, the eighth reliability parameter is an indicator that indicatessuitability of the specific element for an element corresponding to thedirect wave. Higher reliability is obtained as the suitability of thedirect wave for the first incoming wave gets higher, and lowerreliability is obtained as the suitability of the direct wave for thefirst incoming wave gets lower.

The eighth reliability parameter may be calculated on the basis ofconsistency between the respective first incoming waves that aredetected with regard to the plurality of the wireless communicationsections 210. Specifically, the eighth reliability parameter iscalculated on the basis of at least any of reception time and electricpower of the first incoming wave detected with regard to each of theplurality of wireless communication sections 210. Due to the effect ofmultipath, a plurality of wireless signals coming through differentpaths may be combined and received by the wireless communicationsections 210 in a state where the signals are amplified or offset. Next,in the case where ways of amplifying and offsetting the wireless signalsare different between the plurality of wireless communication sections210, different reception times and different electric power values maybe obtained with regard to the first incoming waves between the wirelesscommunication sections 210. When considering that distances between theantennas 211 are short distances that are a half or less of thewavelength λ of the angle estimation signal, a large difference in thereception times and electric power values of the first incoming wavesbetween the wireless communication sections 210 means low suitability ofthe direct waves for the first incoming waves.

In addition, the eighth reliability parameter is calculated in such amanner that the eighth reliability parameter indicates that thesuitability of the direct wave for the first incoming wave gets lower asthe difference in reception time of the first incoming wave between thewireless communication sections 210 gets larger. On the other hand, theeighth reliability parameter is calculated in such a manner that theeighth reliability parameter indicates that the suitability of thedirect waves for the first incoming waves gets higher as the differencein reception time of the first incoming wave between the wirelesscommunication sections 210 gets smaller. In addition, the eighthreliability parameter is calculated in such a manner that the eighthreliability parameter indicates that the suitability of the direct wavefor the first incoming wave gets lower as the difference in electricpower value of the first incoming wave between the wirelesscommunication sections 210 gets larger. On the other hand, the eighthreliability parameter is calculated in such a manner that the eighthreliability parameter indicates that the suitability of the direct wavefor the first incoming wave gets higher as the difference in electricpower value of the first incoming wave between the wirelesscommunication sections 210 gets smaller.

The eighth reliability parameter may be calculated on the basis ofconsistency between positional parameters indicating positions of theportable device 100 estimated on the basis of the respective firstincoming waves detected by the plurality of wireless communicationsections 210. Here, the positional parameters are the angles α and βillustrated in FIG. 3 and the coordinates (x, y, z) illustrated in FIG.4. The positional parameters are estimated on the basis of the firstincoming waves with regard to combinations of the wireless communicationsections 210. In the case where the first incoming waves are the directwaves, same or substantially same results are obtained with regard tothe angles α and β and the coordinates (x, y, z) even if differentcombinations of the wireless communication sections 210 are used forcalculating the angles α and β and the coordinates (x, y, z). However,in the case where the first incoming waves are not the direct waves,different results may be obtained from the different combinations of thewireless communication sections 210 with regard to the angles α and βand the coordinates (x, y, z).

Accordingly, the eighth reliability parameter is calculated in such amanner that the eighth reliability parameter indicates that thesuitability of the direct waves for the first incoming waves gets higheras the difference in positional parameter calculation result betweendifferent combinations of the wireless communication sections 210 getssmaller. For example, the eighth reliability parameter is calculated insuch a manner that the eighth reliability parameter indicates that thesuitability of the direct waves for the first incoming waves gets higheras an error between α_(AC) and α_(BD) gets smaller and as an errorbetween β_(DC) and β_(BA) gets smaller. These errors have been describedabove with regard to the angle estimation process. On the other hand,the eighth reliability parameter is calculated in such a manner that theeighth reliability parameter indicates that the suitability of thedirect waves for the first incoming waves gets lower as the differencein positional parameter calculation result between differentcombinations of the wireless communication sections 210 gets larger. Forexample, the eighth reliability parameter is calculated in such a mannerthat the eighth reliability parameter indicates that the suitability ofdirect waves for the first incoming waves gets lower as an error betweenα_(AC) and α_(BD) gets larger and as an error between β_(DC) and β_(BA)gets larger. These angles have been described above with regard to theangle estimation process.

By using the eighth reliability parameter, it is possible to evaluatethe reliability on the basis of the suitability of the direct waves forthe first incoming waves.

Ninth Reliability Parameter

The ninth reliability parameter is an indicator that indicatessuitability of a situation of receiving the wireless signal. Higherreliability is obtained when the suitability of the situation ofreceiving the wireless signal is higher, and lower reliability isobtained when the suitability of the situation of receiving the wirelesssignal is lower.

The ninth reliability parameter is calculated on the basis of variationof the plurality of first incoming waves that have been acquired bydetecting the first incoming wave multiple times. Specifically, theninth reliability parameter is calculated on the basis of an amount ofstatistics that indicates variation in the plurality of first incomingwaves such as dispersion of the electric power value of the firstincoming waves, and amounts of dispersion and change in the estimatedpositional parameter (distance R, angles α and β, and coordinates (x, y,z)). Note that, the amount of change in the positional parameters meansintegration of a difference between a previously estimated positionalparameter and a currently estimated positional parameter, a differencebetween a maximum value and a minimum value, or the like. As thedispersion and the amount of change get larger, environmental changeincreases in a time period of receiving the wireless signal multipletimes. Therefore, the ninth reliability parameter is calculated in sucha manner that the ninth reliability parameter indicates that thesuitability of a state of receiving a wireless signal gets higher as thedispersion and the amount of change get smaller. On the other hand, theninth reliability parameter is calculated in such a manner that theninth reliability parameter indicates that the suitability of the stateof receiving the wireless signal gets lower as the dispersion and theamount of change get larger. In addition, examples of the amount ofstatistics indicating variation in the plurality of first incoming wavesinclude a phase difference Pd between the first incoming waves, a widthW of the first incoming wave in the time direction, a state of a phase θof the first incoming wave, and an amount of change and dispersion ofSNR of the first incoming wave.

By using the ninth reliability parameter, it is possible to evaluate thereliability on the basis of the suitability of the state of receivingthe wireless signal. Specifically, it is possible to determine thathigher reliability is obtained as environmental change decreases in thetime period of receiving the wireless signal multiple times, and lowerreliability is obtained as the environmental change increases. Inaddition, it is possible to determine that higher reliability isobtained in a low noise situation, and lower reliability is obtained ina high noise situation.

<4.3. Determination of Positional Parameter>

The communication unit 200 (specifically, control section 230) control arepetition process of repeatedly performing a measurement process. Inthe repetition process, the communication unit 200 controls theselection process. Next, the communication unit 200 performs apositional parameter determination process on the basis of the firstincoming waves (that is, specific elements) obtained through therepetition process. Next, details of the measurement process, therepetition process, the selection process, and the positional parameterdetermination process will be described.

(1) Measurement Process

The measurement process includes transmission of a signal from arepresentative wireless communication section, which is a wirelesscommunication section 210 selected from among the plurality of wirelesscommunication sections 210. The master is an example of therepresentative wireless communication section. The second ranging signaltransmitted from the master is an example of the signal transmitted fromthe representative wireless communication section.

The measurement process may include reception of the signal from therepresentative wireless communication section, as a stage prior to thetransmission of the signal from the representative wirelesscommunication section. The first ranging signal transmitted from themaster is an example of the signal received by the representativewireless communication section.

The measurement process includes reception of the signal by theplurality of wireless communication sections 210. The angle estimationsignal, which also serves as the third ranging signal, is an example ofthe respective signals received by the plurality of wirelesscommunication sections 210.

The series of communication may also be referred to as positionestimation communication.

The measurement process includes calculation of a first parameterbelonging to the reliability parameter with regard to at least any ofthe wireless communication sections 210. The reliability parameterserves as an indicator that indicates whether a first incoming wave isappropriate for a processing target. The first incoming wave is a signaldetected as a signal that meets a predetermined detection standard amongsignals received by the wireless communication section 210 through theposition estimation communication. All of the above-describedreliability parameters, that is, the first reliability parameter to theninth reliability parameter are examples of the first parameter. Thecontrol section 230 detects the first incoming wave from a signalreceived by at least any of the wireless communication sections 210, andcalculates the first parameter with regard to the detected firstincoming wave. Such a configuration makes it possible to evaluatewhether the first incoming wave detected in the measurement process isappropriate for a processing target.

Note that, the position estimation communication may be performed onetime in the single measurement process. In other words, thecommunication unit 200 may receive the wireless signals (for example,angle estimation signals that also serve as the third ranging signals)from the plurality of wireless communication sections 210 one time inthe single measurement process. In this case, a single combination ofthe first incoming wave and the reliability parameter can be obtainedthrough the single measurement process. Alternatively, the positionestimation communication may be performed multiple times in the singlemeasurement process. In other words, the communication unit 200 mayreceive the wireless signals (for example, angle estimation signals thatalso serve as the third ranging signals) from the plurality of wirelesscommunication sections 210 multiple times in the single measurementprocess. In this case, a plurality of combinations of the first incomingwave and the reliability parameter can be obtained through the singlemeasurement process.

Typically, after the repetition process, the positional parameterestimation process is performed on the basis of the first incoming wavedetected through the measurement process. Of course, the positionalparameter estimation process may be performed in the measurementprocess. This is because the positional parameter may be used forcalculating the eighth reliability parameter and the ninth reliabilityparameter.

(2) Repetition Process

The communication unit 200 performs the repetition process of repeatedlyperforming the measurement process. Accordingly, the number ofcombinations of the first incoming wave and the first parameter obtainedin the single measurement process is the same as the number ofrepetitions of the measurement process.

(3) Selection Process

The communication unit 200 controls the selection process of selectingthe representative wireless communication section from among theplurality of wireless communication sections 210 each time themeasurement process is repeated in the repetition process. For example,the communication unit 200 selects any wireless communication section210 as the master and performs a first measurement process. Beforeperforming second and subsequent measurement processes, thecommunication unit 200 may select a master again. Such a configurationmakes it possible to perform the measurement process by selecting amaster that is evaluated as an appropriate processing target. Therefore,it is possible to improve accuracy of estimating a positional parameter.

In the selection process, the control unit 200 determines whether or notto change the representative wireless communication section, on thebasis of a second parameter belonging to the reliability parametercalculated with regard to the representative wireless communicationsection. For example, the communication unit 200 may determine to changethe master in the case where the second parameter is calculated withregard to the master and the second parameter indicates lowerreliability than a predetermined threshold. On the other hand, thecommunication unit 200 may determine not to change the master in thecase where the second parameter is calculated with regard to the masterand the second parameter indicates higher reliability than thepredetermined threshold. For another example, the communication unit 200may determine to change the master in the case where reliabilityindicated by a second parameter, which is calculated with regard to themaster, is lower than reliability indicated by a second parameter, whichis calculated with regard to any of slaves. On the other hand, thecommunication unit 200 may determine not to change the master in thecase where reliability indicated by a second parameter, which iscalculated with regard to the master, is higher than reliabilityindicated by second parameters, which are calculated with regard toevery slave. Such a configuration makes it possible to determine whetheror not it is necessary to change the master on the basis of whether ornot the first incoming wave detected with regard to the master isappropriate for the processing target, that is, on the basis ofaccuracies of estimation of possible positional parameters.

In the selection process, the control unit 200 selects a wirelesscommunication section 210 for which a second parameter indicating thatthe wireless communication section 210 is most appropriate for theprocessing target is calculated, as the representative wirelesscommunication section, from among the second parameters calculated withregard to the respective wireless communication sections 210 in the caseof determining to change the representative wireless communicationsection. For example, the communication unit 200 selects a wirelesscommunication section 210 for which a second parameter with the highestreliability is calculated, as the maser, from among the secondparameters calculated with regard to the plurality of wirelesscommunication sections 210. Such a configuration makes it possible toperform a next measurement process while a wireless communicationsection 210 for which a first incoming wave that is most appropriate forthe processing target has been detected is used as the master. Thismakes it possible to improve accuracy of estimating a positionalparameter.

Among the reliability parameters, the second parameter is an indicatorindicating whether the first incoming wave detected with regard to oneof the wireless communication sections 210 is appropriate for theprocessing target. Among the above-described reliability parameters, thefirst reliability parameter to the seventh reliability parameter areexamples of the second parameter. Note that, like the fourth reliabilityparameter, reliability parameters calculated with regard to theplurality of wireless communication sections 210 are deemed to berespective reliability parameters calculated with regard to theplurality of wireless communication sections 210. Such a configurationmakes it possible to determine whether or not it is necessary to changethe master, on the basis of whether or not the first incoming wavedetected with regard to the master is appropriate for the processingtarget.

The first parameter may be the same as the second parameter. In thiscase, the first parameter calculated through the measurement process canbe used as the second parameter in the selection process. Therefore, itis possible to reduce processing load.

Of course, the first parameter may be different from the secondparameter. In this case, the second parameter, which is different fromthe first parameter, is calculated in the selection process.

(4) Positional Parameter Determination Process

The communication unit 200 controls the positional parameterdetermination process on the basis of the first parameter. Thepositional parameter determination process is a process of determining apositional parameter indicating a position of the portable device 100 onthe basis of a plurality of first incoming waves obtained throughrepetition of the measurement process. Specifically, the communicationunit 200 determines the positional parameter of the portable device 100on the basis of the plurality of first incoming waves obtained throughthe position estimation communication performed multiple times in themeasurement process. This makes it possible to determine the positionalparameter without overly depending on each first incoming wave. Such aconfiguration allows the communication unit 200 to determine theposition of the portable device 100 with higher accuracy.

The communication unit 200 determines the positional parameter in thepositional parameter determination process by applying a statisticalprocess based on the first parameter, to the plurality of positionalparameters estimated through the positional parameter estimation processon the basis of the plurality of first incoming waves. Specifically, thecommunication unit 200 estimates the plurality of positional parametersby performing the positional parameter estimation process on theplurality of first incoming waves detected through the repetitionprocess. Next, the communication unit 200 determines, as the positionalparameter, a representative value derived from the plurality ofestimated positional parameters on the basis of the first parameter.

For example, the communication unit 200 may determine the positionalparameter by adopting a positional parameter estimated on the basis of afirst incoming wave corresponding to the first parameter representinghighest reliability. For another example, the communication unit 200 maydetermine the positional parameter by averaging the plurality ofestimated positional parameters through weighted averaging based on thefirst parameter. At this time, a heavier weight is given to a positionalparameter estimated on the basis of the first incoming wavecorresponding to the first parameter representing high reliability, anda lighter weight is given to a positional parameter estimated on thebasis of a first incoming wave corresponding to a first parameterrepresenting low reliability. For another example, the communicationunit 200 may determine the positional parameter by averaging orcalculating a median of a plurality of estimated positional parametersother than positional parameters estimated on the basis of firstincoming waves corresponding to the first parameter representing lowreliability. Note that, such statistical processes may be applicable incombination. Such a configuration makes it possible to determine ahighly accurate positional parameter.

The communication unit 200 may determine a region including the portabledevice 100 in the position determination process. The communication unit200 determines the region including the portable device 100 on the basisof the positional parameter (for example, at least any of a distance R,angles α and β, and coordinates (x, y, z)) determined through theabove-described process. For example, the communication unit 200 maydetermine the region including the portable device 100 among theplurality of regions including the vehicle interior and the vehicleexterior of the vehicle 202. This makes it possible to provide courteousservice such as providing different serves between the case where theuser is in the vehicle interior and the case where the user is in thevehicle exterior. In addition, the communication unit 200 may determinethe region including the portable device 100 after segmenting thevehicle exterior region into a region within a predetermined distancefrom the communication unit 200, and a region that is the predetermineddistance or more away from the communication unit 200.

For example, the determined region including the portable device 100 isused for authentication of the portable device 100. According to thepresent embodiment, it is possible to improve accuracy of the determinedpositional parameters. This makes it possible to prevent erroneousauthentication and improve security.

Note that, sometimes the positional parameter estimation process hasalready been performed in the measurement process because of calculationof the first parameter. In this case, the communication unit 200 doesnot perform the positional parameter estimation process again in thepositional parameter determination process, but may use a positionalparameter estimated through the positional parameter estimation processperformed in the measurement process. On the other hand, in the casewhere the positional parameter estimation process has not been performedin the measurement process, the communication unit 200 performs thepositional parameter estimation process in the positional parameterdetermination process.

The communication unit 200 does not have to determine the positionalparameter in the positional parameter determination process in the casewhere a predetermined condition is satisfied. Hereinafter, such apredetermined condition is also referred to as a suspension condition. Afirst example of the suspension condition is a condition that apredetermined number of first incoming waves corresponding to the firstparameter representing a certain degree of high reliability (reliabilityof a first threshold or more) are not obtained in the measurementprocess. A second example of the suspension condition is a conditionthat a first incoming wave corresponding to the first parameterrepresenting significantly low reliability (reliability of a secondthreshold or less) is obtained in the measurement process. Such aconfiguration makes it possible to avoid a situation of determining apositional parameter with flagrant error, by determining no positionalparameter in a situation where it is expected that accuracy ofdetermining the positional parameter is low.

<4.4. Flow of Process>

FIG. 19 is a flowchart illustrating an example of a flow of a processperformed by the communication unit 200 of the vehicle 202 according tothe present embodiment. According to this flowchart, it is assumed thatthe measurement process is repeated X number of times. In addition, itis assumed that the above-described second example of suspensioncondition is used as the suspension condition in this flowchart.

As illustrated in FIG. 19, the communication unit 200 first performs theposition estimation communication (Step S302). For example, the portabledevice 100 transmits the first ranging signal as described above withreference to FIG. 7. When the first ranging signal is received, themaster of the communication unit 200 transmits the second rangingsignal. When the second ranging signal is received, the portable device100 transmits a signal that serves as both the third ranging signal andthe angle estimation signal. Next, each of the plurality of wirelesscommunication sections 210 receives the signal that serves as both thethird ranging signal and the angle estimation signal. Each of theportable device 100 and the communication unit 200 detects the firstincoming waves from respective signals received through the positionestimation communication. In addition, the control unit 200 may measurea time period T3 from reception time of the first ranging signal totransmission time of the second ranging signal, and a time period T4from transmission time of the second ranging signal to reception time ofthe third ranging signal. In addition, the portable device 100 maymeasure a time period T1 from transmission time of the first rangingsignal to reception time of the second ranging signal and a time periodT2 from reception time of the second ranging signal to transmission timeof the third ranging signal, and then transmit a signal including thetime period T1 and time period T2.

Next, the communication unit 200 calculates the first parameterbelonging to the reliability parameter, on the basis of the firstincoming wave obtained through the position estimation communication(Step S304). For example, the communication unit 200 calculates any ofthe first reliability parameter to the eighth reliability parameter, asthe first parameter.

Next, the communication unit 200 determines whether or not thesuspension condition is satisfied (Step S306). In the case where it isdetermined that the suspension condition is satisfied (YES in StepS306), the communication unit 200 ends the process without determiningthe positional parameter.

On the other hand, in the case where it is determined that thesuspension condition is not satisfied (NO in Step S306), thecommunication unit 200 determines whether or not the position estimationcommunication has been performed X number of times (Step S308).

In the case where it is determined that the position estimationcommunication has not been performed X number of times (NO in StepS308), the communication unit 200 determines whether or not to changethe master on the basis of the second parameter belonging to thereliability parameter, which has been calculated with regard to themaster (Step S310). For example, the communication unit 200 determinesto change the master if the second parameter is calculated with regardto the master and the second parameter indicates lower reliability thana predetermined threshold. If not, the communication unit 200 determinesnot to change the master.

Note that, it is also possible for the communication unit 200 to use thefirst parameter calculated in Step S304, as the second parameter.Alternatively, the communication unit 200 may calculate any of the firstreliability parameter to the seventh reliability parameter, as thesecond parameter separately from the first parameter.

In the case where the communication unit 200 has determined to changethe master (YES in Step S310), the communication unit 200 selects amaster on the basis of the second parameter (Step S312). For example,the communication unit 200 selects, as the maser, a wirelesscommunication section 210 for which a second parameter indicating thehighest reliability is calculated, from among the respective secondparameters calculated with regard to the plurality of wirelesscommunication sections 210. Next, the process returns to Step S302.

On the other hand, in the case where the communication unit 200 hasdetermined not to change the master (NO in Step S310), the processreturns to Step S302.

In the case where it is determined that the position estimationcommunication has been performed X number of times in Step S308 (YES inStep S308), the communication unit 200 determines the positionalparameter of the portable device 100 on the basis of the first parameterand N number of first incoming waves obtained through the positionestimation communication performed X number of times (Step S314).

5. Supplement

Heretofore, preferred embodiments of the present invention have beendescribed in detail with reference to the appended drawings, but thepresent invention is not limited thereto. It should be understood bythose skilled in the art that various changes and alterations may bemade without departing from the spirit and scope of the appended claims.

For example, it is also possible to use a combination of any two or morereliability parameters among the plurality of reliability parametersdescribed in the above embodiment.

For example, the above embodiment has been described on the assumptionthat the CIR is the correlation computation result. However, presentinvention is not limited thereto. For example, the CIR may be thereception signal itself (complex number including IQ components). TheCIR value may be may be the complex number including IQ components,which is the reception signal, the CIR value may be a phase or amplitudeof the reception signal, or the CIR value may be electric power that isa sum of squares of the I component and the Q component of the receptionsignal (or square of amplitude). In this case, the receiver detects thefirst incoming wave from the reception signal. For example, the receivermay use a condition that amplitude or reception electric power of areceived wireless signal exceeds a predetermined threshold for the firsttime, as the predetermined detection standard for detecting the firstincoming wave. In this case, the receiver may detect a signal (morespecifically, sampling point) having amplitude or reception electricpower that exceeds the predetermined threshold for the first time, asthe first incoming wave among reception signals.

For example, in the above-described embodiment, the control section 230calculates the CIR, detects the first incoming wave (that is, specificelement), and estimates the positional parameter. However, the presentinvention is not limited thereto. Any of the above-described processesmay be performed by the wireless communication section 210. For example,each of the plurality of wireless communication sections 210 maycalculate the CIR and detect the first incoming wave on the basis of thereception signal received by each of the plurality of wirelesscommunication sections 210. In addition, the positional parameter may beestimated by the wireless communication section 210 that functions asthe master.

For example, according to the above-described embodiment, thedescription has been given with reference to the example in which theangles α and β are calculated on the basis of antenna array phasedifferences between antennas in a pair. However, the present inventionis not limited thereto. For example, the communication unit 200 maycalculate the angles α and β through beamforming using the plurality ofantennas 211. In this case, the communication unit 200 scans main lobesof the plurality of antennas 211 in all the directions, determines thatthe portable device 100 exists in a direction with largest receptionelectric power, and calculates the angles α and β on the basis of thisdirection.

For example, according to the above-described embodiment, as describedwith reference to FIG. 3, the local coordinate system has been treatedas a coordinate system including coordinate axes parallel to axesconnecting the antennas in the pairs. However, the present invention isnot limited thereto. For example, the local coordinate system may be acoordinate system including coordinate axes that are not parallel to theaxes connecting the antennas in the pairs. In addition, the origin isnot limited to the center of the plurality antennas 211. The localcoordinate system according to the present embodiment may be arbitrarilyset on the basis of arrangement of the plurality of antennas 211 of thecommunication unit 200.

For example, although the example in which the portable device 100serves as the authenticatee and the communication unit 200 serves as theauthenticator has been described in the above embodiment, the presentinvention is not limited thereto. The roles of the portable device 100and the communication unit 200 may be reversed. For example, thepositional parameter may be estimated by the portable device 100. Inaddition, the roles of the portable device 100 and the communicationunit 200 may be switched dynamically. In addition, a plurality of thecommunication units 200 may determine the positional parameters, andperform authentication.

For example, although the example in which the present invention isapplied to the smart entry system has been described in the aboveembodiment, the present invention is not limited thereto. The presentinvention is applicable to any system that estimates the positionalparameter and performs the authentication by transmitting/receivingsignals. For example, the present invention is applicable to a pair ofany two devices selected from a group including portable devices,vehicles, smartphones, drones, houses, home appliances, and the like. Inthis case, one in the pair operates as the authenticator, and the otherin the pair operates as the authenticatee. Note that, the pair mayinclude two device of a same type, or may include two different types ofdevices. In addition, the present invention is applicable to a casewhere a wireless local area network (LAN) router estimates a position ofa smartphone.

For example, in the above embodiment, the standard using UWB has beenexemplified as the wireless communication standard. However, the presentinvention is not limited thereto. For example, it is also possible touse a standard using infrared as the wireless communication standard.

Note that, a series of processes performed by the devices described inthis specification may be achieved by any of software, hardware, and acombination of software and hardware. A program that configures softwareis stored in advance in, for example, a recording medium (non-transitorymedium) installed inside or outside the devices. In addition, forexample, when a computer executes the programs, the programs are readinto random access memory (RAM), and executed by a processor such as aCPU. The recording medium may be a magnetic disk, an optical disc, amagneto-optical disc, flash memory, or the like. Alternatively, theabove-described computer program may be distributed via a networkwithout using the recording medium, for example.

Further, in the present specification, the processes described usingflowcharts are not necessarily executed in the order illustrated in thedrawings. Some processing steps may be executed in parallel. Inaddition, additional processing steps may be employed and someprocessing steps may be omitted.

REFERENCE SIGNS LIST

-   1 system-   100 portable device-   110 wireless communication section-   111 antenna-   120 storage section-   130 control section-   200 communication unit-   202 vehicle-   210 wireless communication section-   211 antenna-   220 storage section-   230 control section

What is claimed is:
 1. A communication device comprising: a plurality ofwireless communication sections, each of which is configured to becapable of wirelessly transmitting and receiving a signal to and fromanother communication device; and a control section configured torepeatedly perform a measurement process including transmission of asignal from a representative wireless communication section that is awireless communication section selected from among the plurality ofwireless communication sections, reception of the signal by theplurality of wireless communication sections, and calculation of a firstparameter belonging to a reliability parameter with regard to at leastany of the wireless communication sections, the reliability parameterserving as an indicator that indicates whether a first incoming wave isappropriate for a processing target, the first incoming wave being asignal detected as a signal that meets a predetermined detectionstandard among the signals received by the wireless communicationsection, control a selection process of selecting the representativewireless communication section from among the plurality of wirelesscommunication sections each time the measurement process is repeated,and control a positional parameter determination process on a basis ofthe first parameter, the positional parameter determination processbeing a process of determining a positional parameter indicating aposition of the other communication device on a basis of a plurality ofthe first incoming waves obtained through repetition of the measurementprocess.
 2. The communication device according to claim 1, wherein, inthe selection process, the control section determines whether or not tochange the representative wireless communication section, on a basis ofa second parameter belonging to the reliability parameter calculatedwith regard to the representative wireless communication section.
 3. Thecommunication device according to claim 2, wherein, in the selectionprocess, the control section selects a wireless communication sectionfor which a second parameter indicating that the wireless communicationsection is most appropriate for the processing target is calculated, asthe representative wireless communication section on a basis of thesecond parameters calculated with regard to the respective wirelesscommunication sections in the case of determining to change therepresentative wireless communication section.
 4. The communicationdevice according to claim 2, wherein, among the reliability parameters,the second parameter is an indicator indicating whether the firstincoming wave detected with regard to one of the wireless communicationsections is appropriate for the processing target.
 5. The communicationdevice according to claim 2, wherein the first parameter is same as thesecond parameter.
 6. The communication device according to claim 1,wherein, in the positional parameter determination process, the controlsection determines the positional parameter by applying a statisticalprocess based on the first parameter, to a plurality of the positionalparameters respectively estimated on a basis of a plurality of the firstincoming waves.
 7. The communication device according to claim 1,wherein the control section does not determine the positional parameterin the positional parameter determination process in the case where apredetermined condition is satisfied.
 8. The communication deviceaccording to claim 1, wherein the positional parameter includes at leastany of a distance to the other communication device from therepresentative wireless communication section, an angle between acoordinate axis and a straight line connecting the other communicationdevice to an origin of a first predetermined coordinate system, andcoordinates of the other communication device in a second predeterminedcoordinate system.
 9. The communication device according to claim 8,wherein the communication device is installed in a vehicle, the othercommunication device is carried by a user of the vehicle, and thecontrol section determines a region including the other communicationdevice among a plurality of regions including a vehicle interior and avehicle exterior of the vehicle, in the positional parameterdetermination process.
 10. A non-transitory computer readable storagemedium having a program that causes a computer for controlling acommunication device including a plurality of wireless communicationsections, each of which is configured to be capable of wirelesslytransmitting and receiving a signal to and from another communicationdevice, to function as a control section configured to repeatedlyperform a measurement process including transmission of a signal from arepresentative wireless communication section that is a wirelesscommunication section selected from among the plurality of wirelesscommunication sections, reception of the signal by the plurality ofwireless communication sections, and calculation of a first parameterbelonging to a reliability parameter with regard to at least any of thewireless communication sections, the reliability parameter serving as anindicator that indicates whether a first incoming wave is appropriatefor a processing target, the first incoming wave being a signal detectedas a signal that meets a predetermined detection standard among thesignals received by the wireless communication section, control aselection process of selecting the representative wireless communicationsection from among the plurality of wireless communication sections eachtime the measurement process is repeated, and control a positionalparameter determination process on a basis of the first parameter, thepositional parameter determination process being a process ofdetermining a positional parameter indicating a position of the othercommunication device on a basis of a plurality of the first incomingwaves obtained through repetition of the measurement process.